Don't crash if Modal related commands are called before root is set

We should also support showingModals on Android regardless of root

lib/android/app/src/main/java/com/reactnativenavigation/viewcontrollers/modal/ModalPresenter.java

 import android.animation.Animator;
 import android.animation.AnimatorListenerAdapter;
 import android.support.annotation.NonNull;
+import android.support.annotation.Nullable;
 import android.view.ViewGroup;
 
 import com.reactnativenavigation.anim.ModalAnimator;
 
 public class ModalPresenter {
 
-    private ViewGroup content;
+    @Nullable private ViewGroup content;
     private ModalAnimator animator;
     private Options defaultOptions = new Options();
     private EventEmitter eventEmitter;
     }
 
     public void showModal(ViewController toAdd, ViewController toRemove, CommandListener listener) {
+        if (content == null) {
+            listener.onError("Could not show modal before setRoot is called");
+            return;
+        }
         Options options = toAdd.resolveCurrentOptions(defaultOptions);
         toAdd.setWaitForRender(options.animations.showModal.waitForRender);
         content.addView(toAdd.getView());
     }
 
     public void dismissTopModal(ViewController toDismiss, @NonNull ViewController toAdd, CommandListener listener) {
+        if (content == null) {
+            listener.onError("Could not dismiss modal before setRoot is called");
+            return;
+        }
         toAdd.attachView(content, 0);
         dismissModal(toDismiss, listener);
     }
 
     public void dismissModal(ViewController toDismiss, CommandListener listener) {
+        if (content == null) {
+            listener.onError("Could not dismiss modal before setRoot is called");
+            return;
+        }
         if (toDismiss.options.animations.dismissModal.enable.isTrueOrUndefined()) {
             animator.dismiss(toDismiss.getView(), toDismiss.options.animations.dismissModal, new AnimatorListenerAdapter() {
                 @Override

lib/android/app/src/test/java/com/reactnativenavigation/viewcontrollers/modal/ModalPresenterTest.java

 import org.json.JSONException;
 import org.json.JSONObject;
 import org.junit.Test;
+import org.mockito.Mockito;
 
 import static org.assertj.core.api.Java6Assertions.assertThat;
 import static org.mockito.ArgumentMatchers.any;
         verifyZeroInteractions(animator);
     }
 
+    @Test
+    public void showModal_rejectIfContentIsNull() {
+        uut.setContentLayout(null);
+        CommandListenerAdapter listener = Mockito.mock(CommandListenerAdapter.class);
+        uut.showModal(modal1, modal2, listener);
+        verify(listener).onError(any());
+    }
+
     @Test
     public void dismissModal_animatesByDefault() {
         disableShowModalAnimation(modal1);
         assertThat(root.getView().getParent()).isNotNull();
         verify(root, times(0)).onViewDisappear();
     }
+
+    @Test
+    public void dismissTopModal_rejectIfContentIsNull() {
+        uut.setContentLayout(null);
+        CommandListenerAdapter listener = Mockito.mock(CommandListenerAdapter.class);
+        uut.dismissTopModal(modal1, modal2, listener);
+        verify(listener).onError(any());
+    }
+
+    @Test
+    public void dismissModal_rejectIfContentIsNull() {
+        uut.setContentLayout(null);
+        CommandListenerAdapter listener = Mockito.mock(CommandListenerAdapter.class);
+        uut.dismissModal(modal1, listener);
+        verify(listener).onError(any());
+    }
 }