Log SoftError when there is not EventDispatcher associated to UIManager

Summary:
This diff logs a SoftError when there is not EventDispatcher associated to UIManager

The app will crash in Debug mode, this will not affect production users
changelog: [internal]

Reviewed By: JoshuaGross

Differential Revision: D25859546

fbshipit-source-id: 8045bcd67f613ea6286f30fe6f3c66113c700b0b

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

@@ -111,7 +111,17 @@ public class UIManagerHelper {
       return ((EventDispatcherProvider) context).getEventDispatcher();
     }
     UIManager uiManager = getUIManager(context, uiManagerType, false);
-    return uiManager == null ? null : (EventDispatcher) uiManager.getEventDispatcher();
+    if (uiManager == null) {
+      return null;
+    }
+    EventDispatcher eventDispatcher = (EventDispatcher) uiManager.getEventDispatcher();
+    if (eventDispatcher == null) {
+      ReactSoftException.logSoftException(
+          "UIManagerHelper",
+          new IllegalStateException(
+              "Cannot get EventDispatcher for UIManagerType " + uiManagerType));
+    }
+    return eventDispatcher;
   }
 
   /**