ignore dropView method when view is null (facebook#24347)

Summary:
In facebook#20288, we solved `com.facebook.react.uimanager.NativeViewHierarchyManager.dropView (NativeViewHierarchyManager.java:536)` by adding codes that prevent to run method on `view` object when `view.getId()` is null.

But if `view` is null, `view.getId()` can make error about NullPointerException. So, I think `dropView` method  needs to check if `view` is null.

If you need more information, Please read facebook#24346.

[Android] [Fixed] - Ignore dropView method when view is null.
Pull Request resolved: facebook#24347

Differential Revision: D14833822

Pulled By: cpojer

fbshipit-source-id: 88b6a05090ea8e8d6737c1f10b40e93649fab151

ReactAndroid/src/main/java/com/facebook/react/uimanager/NativeViewHierarchyManager.java

@@ -592,6 +592,10 @@ protected synchronized final void addRootViewGroup(int tag, View view) {
    */
   protected synchronized void dropView(View view) {
     UiThreadUtil.assertOnUiThread();
+    if (view == null) {
+      // Ignore this drop operation when view is null.
+      return;
+    }
     if (mTagsToViewManagers.get(view.getId()) == null) {
       // This view has already been dropped (likely due to a threading issue caused by async js
       // execution). Ignore this drop operation.