Fix potential NPE in UIViewOperationQueue.

Summary:
We were noticing the following crash in our application, that was occurring fairly often, but still hard to reproduce:

```
12-12 10:37:35.342: E/AndroidRuntime(9064): FATAL EXCEPTION: main
12-12 10:37:35.342: E/AndroidRuntime(9064): Process: com.bloomberg.android.plus, PID: 9064
12-12 10:37:35.342: E/AndroidRuntime(9064): java.lang.NullPointerException: Attempt to invoke interface method 'void com.facebook.react.uimanager.UIViewOperationQueue$UIOperation.execute()' on a null object reference
12-12 10:37:35.342: E/AndroidRuntime(9064): 	at com.facebook.react.uimanager.UIViewOperationQueue$2.run(UIViewOperationQueue.java:782)
12-12 10:37:35.342: E/AndroidRuntime(9064): 	at com.facebook.react.uimanager.UIViewOperationQueue.flushPendingBatches(UIViewOperationQueue.java:829)
12-12 10:37:35.342: E/AndroidRuntime(9064): 	at com.facebook.react.uimanager.UIViewOperationQueue.access$1500(UIViewOperationQueue.java:44)
12-12 10:37:35.342: E/AndroidRuntime(9064): 	at com.facebook.react.uimanager.UIViewOperationQ
Closes facebook#11428

Differential Revision: D4487841

Pulled By: astreet

fbshipit-source-id: ae49ef77967edea7514cbf40cb533c4b63fd34ae

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

@@ -25,6 +25,7 @@
 import com.facebook.react.bridge.ReactContext;
 import com.facebook.react.bridge.ReadableArray;
 import com.facebook.react.bridge.ReadableMap;
+import com.facebook.react.bridge.SoftAssertions;
 import com.facebook.react.bridge.UiThreadUtil;
 import com.facebook.react.uimanager.debug.NotThreadSafeViewHierarchyUpdateDebugListener;
 import com.facebook.systrace.Systrace;
@@ -589,6 +590,7 @@ public void run() {
    * subclass to support UIOperations not provided by UIViewOperationQueue.
    */
   protected void enqueueUIOperation(UIOperation operation) {
+    SoftAssertions.assertNotNull(operation);
     mOperations.add(operation);
   }