[Share] Fix crash from null tab in menu handler

Bug: 1083851
Change-Id: I569c5dc14de4d6590cbd96901ecae7b505dbdec2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2236248Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Patrick Noland <pnoland@chromium.org>
Commit-Queue: Jeffrey Cohen <jeffreycohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776557}

chrome/android/java/src/org/chromium/chrome/browser/ui/RootUiCoordinator.java

@@ -353,9 +353,12 @@ public class RootUiCoordinator
      */
     @VisibleForTesting
     public void onShareMenuItemSelected(final boolean shareDirectly, final boolean isIncognito) {
-        if (mShareDelegateSupplier.get() == null) return;
+        ShareDelegate shareDelegate = mShareDelegateSupplier.get();
+        Tab tab = mActivityTabProvider.get();
 
-        mShareDelegateSupplier.get().share(mActivityTabProvider.get(), shareDirectly);
+        if (shareDelegate == null || tab == null) return;
+
+        shareDelegate.share(tab, shareDirectly);
     }
 
     // MenuOrKeyboardActionHandler implementation