fix(android): check null activity

If activity is null at initialization, the app crashes. This change checks for null activity and throw react native error, which prevents app crash.

android/src/main/java/com/brentvatne/exoplayer/ReactExoplayerView.java

@@ -573,6 +573,12 @@ class ReactExoplayerView extends FrameLayout implements
                                     return;
                                 }
                                     
+                                if (activity == null) {
+                                    Log.e("ExoPlayer Exception", "Failed to initialize Player!");
+                                    eventEmitter.error("Failed to initialize Player!", new Exception("Current Activity is null!"), "1001");
+                                    return;
+                                }
+
                                 // Initialize handler to run on the main thread
                                 activity.runOnUiThread(new Runnable() {
                                     public void run() {