Your theme isn't set to use Theme.AppCompat or Theme.MaterialComponents.

[RajatKhoware]

Describe the bug
When I try to make payment using Stripe i got to see these errors the payment intent was created but this problem occurs when we try to make payment

PlatformException(flutter_stripe initialization failed, The plugin failed to initialize:
I/flutter ( 5596): Your Main Activity class com.example.new_user_side.MainActivity is not a subclass FlutterFragmentActivity.
I/flutter ( 5596): Please make sure you follow all the steps detailed inside the README: https://github.com/flutter-stripe/flutter_stripe#android
I/flutter ( 5596): If you continue to have trouble, follow this discussion to get some support #538, null, null)
E/flutter ( 5596): [ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: Exception: PlatformException(flutter_stripe initialization failed, The plugin failed to initialize:

MainActivity class code
package com.example.new_user_side import io.flutter.embedding.android.FlutterActivity class MainActivity: FlutterActivity() { }

AndroidMainfest.xml

    package="com.example.new_user_side">
    <uses-permission android:name="android.permission.INTERNET"/>
    <!-- Permissions options for the `storage` group -->
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.READ_MEDIA_IMAGES" />
    <uses-permission android:name="android.permission.READ_MEDIA_VIDEO" />
    <uses-permission android:name="android.permission.READ_MEDIA_AUDIO" />

    <!-- Permissions options for the `camera` group -->
    <uses-permission android:name="android.permission.CAMERA"/>
    <application
        android:label="new_user_side"
        android:name="${applicationName}"
        android:icon="@mipmap/ic_launcher">
        <activity
            android:name=".MainActivity"
            android:exported="true"
            android:launchMode="singleTop"
            android:theme="@style/LaunchTheme"
            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
            android:hardwareAccelerated="true"
            android:windowSoftInputMode="adjustResize">
            <!-- Specifies an Android theme to apply to this Activity as soon as
                 the Android process has started. This theme is visible to the user
                 while the Flutter UI initializes. After that, this theme continues
                 to determine the Window background behind the Flutter UI. -->
            <meta-data
              android:name="io.flutter.embedding.android.NormalTheme"
              android:resource="@style/NormalTheme"
              />
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
        <!-- Don't delete the meta-data below.
             This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
        <meta-data
            android:name="flutterEmbedding"
            android:value="2" />
    </application>
</manifest>

build.gradle

def localProperties = new Properties()
def localPropertiesFile = rootProject.file('local.properties')
if (localPropertiesFile.exists()) {
    localPropertiesFile.withReader('UTF-8') { reader ->
        localProperties.load(reader)
    }
}

def flutterRoot = localProperties.getProperty('flutter.sdk')
if (flutterRoot == null) {
    throw new GradleException("Flutter SDK not found. Define location with flutter.sdk in the local.properties file.")
}

def flutterVersionCode = localProperties.getProperty('flutter.versionCode')
if (flutterVersionCode == null) {
    flutterVersionCode = '1'
}

def flutterVersionName = localProperties.getProperty('flutter.versionName')
if (flutterVersionName == null) {
    flutterVersionName = '1.0'
}

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'
apply from: "$flutterRoot/packages/flutter_tools/gradle/flutter.gradle"


def keystoreProperties = new Properties()
def keystorePropertiesFile = rootProject.file('key.properties')
if (keystorePropertiesFile.exists()) {
   keystoreProperties.load(new FileInputStream(keystorePropertiesFile))
}

android {
    namespace "com.example.new_user_side"
    compileSdkVersion 33
    ndkVersion flutter.ndkVersion

    compileOptions {
        sourceCompatibility JavaVersion.VERSION_1_8
        targetCompatibility JavaVersion.VERSION_1_8
    }

    kotlinOptions {
        jvmTarget = '1.8'
    }

    sourceSets {
        main.java.srcDirs += 'src/main/kotlin'
    }

    defaultConfig {
        // TODO: Specify your own unique Application ID (https://developer.android.com/studio/build/application-id.html).
        applicationId "com.example.new_user_side"
        // You can update the following values to match your application needs.
        // For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration.
        minSdkVersion 21
        targetSdkVersion 30
        versionCode flutterVersionCode.toInteger()
        versionName flutterVersionName
    }

     signingConfigs {
       release {
           keyAlias keystoreProperties['keyAlias']
           keyPassword keystoreProperties['keyPassword']
           storeFile keystoreProperties['storeFile'] ? file(keystoreProperties['storeFile']) : null
           storePassword keystoreProperties['storePassword']
       }
   }
   buildTypes {
       release {
           signingConfig signingConfigs.release
       }
   }
}

flutter {
    source '../..'
}

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    implementation platform('com.google.firebase:firebase-bom:32.1.0')
}

I am using
flutter_stripe: ^9.2.0

[RajatKhoware]

In MainActivity.kt file i have change this
package com.example.new_user_side import io.flutter.embedding.android.FlutterActivity class MainActivity: FlutterActivity() { }

to this
package com.example.new_user_side import io.flutter.embedding.android.FlutterActivity import io.flutter.embedding.android.FlutterFragmentActivity class MainActivity: FlutterFragmentActivity() { }

now that error was gone and getting new error how to fix it now
PlatformException(flutter_stripe initialization failed, The plugin failed to initialize: I/flutter (11719): Your theme isn't set to use Theme.AppCompat or Theme.MaterialComponents.
and this is androidmainfest.xml file

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.new_user_side">
    <uses-permission android:name="android.permission.INTERNET"/>
    <!-- Permissions options for the `storage` group -->
    <uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"/>
    <uses-permission android:name="android.permission.READ_MEDIA_IMAGES" />
    <uses-permission android:name="android.permission.READ_MEDIA_VIDEO" />
    <uses-permission android:name="android.permission.READ_MEDIA_AUDIO" />

    <!-- Permissions options for the `camera` group -->
    <uses-permission android:name="android.permission.CAMERA"/>
    <application
        android:label="new_user_side"
        android:name="${applicationName}"
        android:icon="@mipmap/ic_launcher">
        <activity
            android:name=".MainActivity"
            android:launchMode="singleTop"
            android:theme="@style/LaunchTheme"
            android:configChanges="orientation|keyboardHidden|keyboard|screenSize|smallestScreenSize|locale|layoutDirection|fontScale|screenLayout|density|uiMode"
            android:hardwareAccelerated="true"
            android:windowSoftInputMode="adjustResize">
            <!-- Specifies an Android theme to apply to this Activity as soon as
                 the Android process has started. This theme is visible to the user
                 while the Flutter UI initializes. After that, this theme continues
                 to determine the Window background behind the Flutter UI. -->
            <meta-data
              android:name="io.flutter.embedding.android.NormalTheme"
              android:resource="@style/NormalTheme"
              />
             <!-- Displays an Android View that continues showing the launch screen
                 Drawable until Flutter paints its first frame, then this splash
                 screen fades out. A splash screen is useful to avoid any visual
                 gap between the end of Android's launch screen and the painting of
                 Flutter's first frame. -->
            <meta-data
              android:name="io.flutter.embedding.android.SplashScreenDrawable"
              android:resource="@drawable/launch_background"
              />  
            <intent-filter>
                <action android:name="android.intent.action.MAIN"/>
                <category android:name="android.intent.category.LAUNCHER"/>
            </intent-filter>
        </activity>
        <!-- Don't delete the meta-data below.
             This is used by the Flutter tool to generate GeneratedPluginRegistrant.java -->
        <meta-data
            android:name="flutterEmbedding"
            android:value="2" />
    </application>
</manifest>

[jonasbark]

Check and follow closely:
https://github.com/flutter-stripe/flutter_stripe#android