android: fix NPE when handling onHostPause

If the Activity is put into the background before the ReactContext is created we
get an NPE here. While the window might be short, it's thechnically possible to
hit this, as our Crashlytics reports show.

android/sdk/src/main/java/org/jitsi/meet/sdk/JitsiMeetActivityDelegate.java

             // https://github.com/facebook/react-native/blob/df4e67fe75d781d1eb264128cadf079989542755/ReactAndroid/src/main/java/com/facebook/react/ReactInstanceManager.java#L512
             // Why this happens is a mystery wrapped in an enigma.
             ReactContext reactContext = reactInstanceManager.getCurrentReactContext();
-            if (activity == reactContext.getCurrentActivity()) {
+            if (reactContext != null && activity == reactContext.getCurrentActivity()) {
                 reactInstanceManager.onHostPause(activity);
             }
         }