[BMO] Actually check the weakptr in chrome://apps uninstall

R=calamity@chromium.org, loyso@chromium

Fixed: 1070185
Change-Id: I84aafb1203586e272bb0d68d8f0aed49f0d7f827
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2147805
Auto-Submit: Daniel Murphy <dmurph@chromium.org>
Commit-Queue: calamity <calamity@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#759127}

chrome/browser/ui/webui/ntp/app_launcher_handler.cc

@@ -865,7 +865,8 @@ void AppLauncherHandler::HandleUninstallApp(const base::ListValue* args) {
         [](base::WeakPtr<AppLauncherHandler> app_launcher_handler,
            bool success) {
           LOCAL_HISTOGRAM_BOOLEAN("Apps.Launcher.UninstallSuccess", success);
-          app_launcher_handler->CleanupAfterUninstall();
+          if (app_launcher_handler)
+            app_launcher_handler->CleanupAfterUninstall();
         },
         weak_ptr_factory_.GetWeakPtr());