Fix closeTab crash

This is the top crash in fabric here:
https://fabric.io/brave6/android/apps/com.linkbubble.playstore/issues/55c93db6e0d514e5d6e4b8ce

For wahtever reason the view is sometimes null when closing a tab. This
just checks to make sure it's not null.  Some of the checks that call
into closeTab already did null checks, the one that was crashing did
not. I thought it was better to just have the check inside the function.

Application/LinkBubble/src/main/java/com/linkbubble/MainController.java

@@ -914,6 +914,10 @@ public boolean closeTab(TabView tabView, boolean animateOff, boolean canShowUndo
 
     public boolean closeTab(TabView tabView, Constant.BubbleAction action, boolean animateOff, boolean canShowUndoPrompt) {
 
+        if (tabView == null) {
+            return false;
+        }
+
         // If the tab is already closing, do nothing. Otherwise we could end up in a weird state,
         // where we attempt to show multiple prompts and crashing upon tab restore.
         if (tabView.mIsClosing == true) {