Fix VM image selection sychronize issue (#4568)

* Fix VM image selection sychronize issue

* Fix checkstyle

* Fix method order

* Clear all sub combobox on refreshing

* Resolve comments

PluginsAndFeatures/azure-toolkit-for-intellij/src/com/microsoft/intellij/util/RxJavaUtils.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) Microsoft Corporation
+ *
+ * All rights reserved.
+ *
+ * MIT License
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated
+ * documentation files (the "Software"), to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and
+ * to permit persons to whom the Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all copies or substantial portions of
+ * the Software.
+ *
+ * THE SOFTWARE IS PROVIDED *AS IS*, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO
+ * THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
+ * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package com.microsoft.intellij.util;
+
+import rx.Subscription;
+
+public class RxJavaUtils {
+    public static void unsubscribeSubscription(Subscription subscription) {
+        if (subscription != null && !subscription.isUnsubscribed()) {
+            subscription.unsubscribe();
+        }
+    }
+}