Merge pull request #528 from rmrs/rumors

Workaround for null reference of database reference off

android/src/main/java/io/invertase/firebase/database/RNFirebaseDatabase.java

@@ -416,12 +416,14 @@ public class RNFirebaseDatabase extends ReactContextBaseJavaModule {
   @ReactMethod
   public void off(String key, String eventRegistrationKey) {
     RNFirebaseDatabaseReference nativeRef = references.get(key);
+    if (nativeRef != null) {
       nativeRef.removeEventListener(eventRegistrationKey);
 
       if (!nativeRef.hasListeners()) {
         references.remove(key);
       }
     }
+  }
 
   /*
    * INTERNALS/UTILS