Fix ReactInstanceManager unmountApplication

Reviewed By: mdvacca

Differential Revision: D7945746

fbshipit-source-id: 0c2eed9a623e442fa4a4ff29ffa01f025466c9b6

ReactAndroid/src/main/java/com/facebook/react/ReactRootView.java

@@ -423,6 +423,7 @@ public class ReactRootView extends SizeMonitoringFrameLayout
   public void unmountReactApplication() {
     if (mReactInstanceManager != null && mIsAttachedToInstance) {
       mReactInstanceManager.detachRootView(this);
+      mReactInstanceManager = null;
       mIsAttachedToInstance = false;
     }
     mShouldLogContentAppeared = false;

ReactAndroid/src/test/java/com/facebook/react/RootViewTest.java

@@ -195,4 +195,15 @@ public class RootViewTest {
         MotionEvent.obtain(50, new Date().getTime(), MotionEvent.ACTION_HOVER_MOVE, 0, 0, 0));
     verifyNoMoreInteractions(eventDispatcher);
   }
+
+  @Test
+  public void testRemountApplication() {
+    ReactInstanceManager instanceManager = mock(ReactInstanceManager.class);
+
+    ReactRootView rootView = new ReactRootView(mReactContext);
+
+    rootView.startReactApplication(instanceManager, "");
+    rootView.unmountReactApplication();
+    rootView.startReactApplication(instanceManager, "");
+  }
 }