[Messages] Fix NPE when modal dialog manager supplier offers a null

When AsyncInitializationActivity is destroyed, modalDialogManagerSupplier
offers a null object, which cause the mediator a NPE.

Bug: 1169413, 1163300
Change-Id: I858ffc2aab2fcedabb792087780072808789f76e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2644064Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Theresa  <twellington@chromium.org>
Auto-Submit: Lijin Shen <lazzzis@google.com>
Cr-Commit-Position: refs/heads/master@{#846192}

chrome/android/java/src/org/chromium/chrome/browser/messages/ChromeMessageQueueMediator.java

@@ -214,6 +214,7 @@ public class ChromeMessageQueueMediator implements MessageQueueDelegate {
     }
 
     private void setTabModelSelector(TabModelSelector tabModelSelector) {
+        if (tabModelSelector == null) return;
         if (mTabModelSelector != null) {
             mTabModelSelector.getTabModelFilterProvider().removeTabModelFilterObserver(
                     mTabModelObserver);
@@ -223,6 +224,7 @@ public class ChromeMessageQueueMediator implements MessageQueueDelegate {
     }
 
     private void setModalDialogManager(ModalDialogManager modalDialogManager) {
+        if (modalDialogManager == null) return;
         if (mModalDialogManager != null) {
             mModalDialogManager.removeObserver(mModalDialogManagerObserver);
         }