unmount fix

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/ViewController.java

 	}
 
 	public void onDestroy() {
-		view.getViewTreeObserver().removeOnGlobalLayoutListener(this);
-		view = null;
+		if (view != null) {
+			view.getViewTreeObserver().removeOnGlobalLayoutListener(this);
+		}
+		if (isShown) {
+			isShown = false;
+			onDisappear();
+		}
 	}
 
 	@Override

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/ViewControllerTest.java

 import org.junit.Test;
 import org.robolectric.Shadows;
 
-import java.lang.reflect.Field;
-
 import static org.assertj.core.api.Java6Assertions.assertThat;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.spy;
 
 	@Test
 	public void onDestroy_RemovesGlobalLayoutListener() throws Exception {
+		new SimpleViewController(activity, "ensureNotNull").onDestroy();
+
 		ViewController spy = spy(uut);
 		View view = spy.getView();
 		Shadows.shadowOf(view).setMyParent(mock(ViewParent.class));
 	}
 
 	@Test
-	public void onDestroy_NullifiesTheView() throws Exception {
-		assertThat(uut.getView()).isNotNull();
-		uut.onDestroy();
-		Field field = ViewController.class.getDeclaredField("view");
-		field.setAccessible(true);
-		assertThat(field.get(uut)).isNull();
+	public void onDestroy_CallsOnDisappearIfNeeded() throws Exception {
+		ViewController spy = spy(uut);
+		Shadows.shadowOf(spy.getView()).setMyParent(mock(ViewParent.class));
+		Assertions.assertThat(spy.getView()).isShown();
+		spy.getView().getViewTreeObserver().dispatchOnGlobalLayout();
+		verify(spy, times(1)).onAppear();
+
+		spy.onDestroy();
+
+		verify(spy, times(1)).onDisappear();
 	}
 }