fix(messaging, android): stop app from crashing when DartCallback or Context is null

packages/firebase_messaging/firebase_messaging/android/src/main/java/io/flutter/plugins/firebase/messaging/FlutterFirebaseMessagingBackgroundExecutor.java

@@ -55,6 +55,10 @@ public class FlutterFirebaseMessagingBackgroundExecutor implements MethodCallHan
    */
   public static void setCallbackDispatcher(long callbackHandle) {
     Context context = ContextHolder.getApplicationContext();
+    if (context == null) {
+      Log.e(TAG, "Context is null, cannot continue.");
+      return;
+    }
     SharedPreferences prefs =
         context.getSharedPreferences(FlutterFirebaseMessagingUtils.SHARED_PREFERENCES_KEY, 0);
     prefs.edit().putLong(CALLBACK_HANDLE_KEY, callbackHandle).apply();
@@ -176,6 +180,12 @@ public void startBackgroundIsolate(long callbackHandle, FlutterShellArgs shellAr
                   // lookup will fail.
                   FlutterCallbackInformation flutterCallback =
                       FlutterCallbackInformation.lookupCallbackInformation(callbackHandle);
+
+                  if (flutterCallback == null) {
+                    Log.e(TAG, "Failed to find registered callback");
+                    return;
+                  }
+
                   DartExecutor executor = backgroundFlutterEngine.getDartExecutor();
                   initializeMethodChannel(executor);
 

packages/firebase_messaging/firebase_messaging/android/src/main/java/io/flutter/plugins/firebase/messaging/FlutterFirebaseMessagingPlugin.java

@@ -87,6 +87,7 @@ private void initInstance(BinaryMessenger messenger) {
 
   @Override
   public void onAttachedToEngine(FlutterPluginBinding binding) {
+    ContextHolder.setApplicationContext(binding.getApplicationContext());
     initInstance(binding.getBinaryMessenger());
   }
 

packages/firebase_messaging/firebase_messaging/android/src/main/java/io/flutter/plugins/firebase/messaging/FlutterFirebaseMessagingReceiver.java

@@ -20,7 +20,12 @@ public class FlutterFirebaseMessagingReceiver extends BroadcastReceiver {
   public void onReceive(Context context, Intent intent) {
     Log.d(TAG, "broadcast received for message");
     if (ContextHolder.getApplicationContext() == null) {
-      ContextHolder.setApplicationContext(context.getApplicationContext());
+      Context aContext = context;
+      if (context.getApplicationContext() != null) {
+        aContext = context.getApplicationContext();
+      }
+
+      ContextHolder.setApplicationContext(aContext);
     }
 
     if (intent.getExtras() == null) {