Feed back for empty stack, experimental fix for react state cleared when view pushed down the stack.

android/app/src/main/java/com/reactnativenavigation/activities/BaseReactActivity.java

 
     public abstract String getActiveNavigatorID();
 
+    public abstract int getScreenStackSize();
+
     @Override
     public boolean onCreateOptionsMenu(Menu menu) {
         mMenu = menu;
 
     @Override
     public void onBackPressed() {
-
-        if(pop(getActiveNavigatorID()) == null) {
+        if(getScreenStackSize() > 1){
+            pop(getActiveNavigatorID());
+        }
+        else{
             if (mReactInstanceManager != null)
                 mReactInstanceManager.onBackPressed();
             else

android/app/src/main/java/com/reactnativenavigation/activities/RootActivity.java

         return null;
         //TODO
     }
+
+    @Override
+    public int getScreenStackSize() {
+        return 0;
+        //TODO
+    }
 }

android/app/src/main/java/com/reactnativenavigation/activities/SingleScreenActivity.java

     public String getActiveNavigatorID() {
         return navID;
     }
+
+    @Override
+    public int getScreenStackSize() {
+        return screenStack.getStackSize();
+    }
 }

android/app/src/main/java/com/reactnativenavigation/activities/TabActivity.java

     public String getActiveNavigatorID() {
         return adapter.getNavID(mViewPager.getCurrentItem());
     }
+
+    @Override
+    public int getScreenStackSize() {
+        return adapter.getStackSizeForNavigatorId(getActiveNavigatorID());
+    }
 }

android/app/src/main/java/com/reactnativenavigation/adapters/ViewPagerAdapter.java

     public String getNavID(int position) {
         return navIDs.get(position);
     }
+
+    public int getStackSizeForNavigatorId(String activeNavigatorID) {
+        return stacksByNavId.get(activeNavigatorID).getStackSize();
+    }
 }

android/app/src/main/java/com/reactnativenavigation/views/ScreenStack.java

         RctView view = new RctView(reactActivity, mReactInstanceManager, screen);
         addView(view, MATCH_PARENT, MATCH_PARENT);
         if(oldView!=null) {
-            removeView(oldView);
+            oldView.setVisibility(GONE);
         }
         stack.push(new ScreenView(screen, view));
     }
 
     public Screen pop(){
-        //TODO maybe return null if size is 1?
         if(stack.isEmpty()) {
             return null;
         }
         ScreenView popped = stack.pop();
         if(!stack.isEmpty()) {
-            addView(stack.peek().view, 0);
+            View view = stack.peek().view;
+            if(view.getParent() == null)
+                addView(stack.peek().view, 0);
+            else {
+                view.setVisibility(VISIBLE);
+            }
         }
         removeView(popped.view);
         return popped.screen;