Fix NPE bug for instance in mode

core/src/main/java/org/lflang/graph/InstantiationGraph.java

@@ -32,6 +32,7 @@
 import org.eclipse.emf.ecore.resource.Resource;
 import org.lflang.ast.ASTUtils;
 import org.lflang.lf.Instantiation;
+import org.lflang.lf.Mode;
 import org.lflang.lf.Model;
 import org.lflang.lf.Reactor;
 import org.lflang.lf.ReactorDecl;
@@ -154,6 +155,12 @@ private void buildGraph(final Instantiation instantiation, final Set<Instantiati
         for (final Instantiation inst : reactor.getInstantiations()) {
           this.buildGraph(inst, visited);
         }
+        // Also have to look for instantiations inside modes.
+        for (final Mode mode : reactor.getModes()) {
+          for (final Instantiation inst : mode.getInstantiations()) {
+            this.buildGraph(inst, visited);
+          }
+        }
       }
     }
   }

test/C/src/ImportInstanceInMode.lf

@@ -0,0 +1,11 @@
+// Test that importing from a file with a reactor that is only instantiated in
+// a mode does not trigger a NPE.
+target C {
+  timeout: 0 s
+}
+
+import B from "lib/InstanceInMode.lf"
+
+main reactor {
+  b = new B()
+}

test/C/src/lib/InstanceInMode.lf

@@ -0,0 +1,14 @@
+// Define a reactor that is only instantiated inside a mode.
+target C
+
+reactor U {
+  reaction(startup) {=  =}
+}
+
+reactor B {
+  reaction(startup) {=  =}
+
+  initial mode A {
+    u = new U()
+  }
+}