Handle NPE when showing Overlay before setRoot is called

Rejecting the promise is a temporary solution, Overlay should not be coupled to root.

lib/android/app/src/main/java/com/reactnativenavigation/presentation/OverlayManager.java

@@ -1,5 +1,6 @@
 package com.reactnativenavigation.presentation;
 
+import android.support.annotation.Nullable;
 import android.view.ViewGroup;
 
 import com.reactnativenavigation.utils.CommandListener;
@@ -10,7 +11,11 @@ import java.util.HashMap;
 public class OverlayManager {
     private final HashMap<String, ViewController> overlayRegistry = new HashMap<>();
 
-    public void show(ViewGroup root, ViewController overlay, CommandListener listener) {
+    public void show(@Nullable ViewGroup root, ViewController overlay, CommandListener listener) {
+        if (root == null) {
+            listener.onError("Can't show Overlay before setRoot is called. This will be resolved in #3899");
+            return;
+        }
         overlayRegistry.put(overlay.getId(), overlay);
         overlay.setOnAppearedListener(() -> listener.onSuccess(overlay.getId()));
         root.addView(overlay.getView());