Add TabModelSelector null-check in #onActivityHidden

Bug: 1090031,1083917
Change-Id: Ie4096f28387f3a9214806179de5a04af06063a7e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225268Reviewed-by: Ted Choc <tedchoc@chromium.org>
Commit-Queue: Brandon Wylie <wylieb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#774281}

chrome/android/java/src/org/chromium/chrome/browser/ChromeActivity.java

@@ -802,7 +802,8 @@ public abstract class ChromeActivity<C extends ChromeActivityComponent>
         VrModuleProvider.getDelegate().onActivityHidden(this);
 
         Tab tab = getActivityTab();
-        if (!mTabModelSelector.isReparentingInProgress() && tab != null) {
+        if (mTabModelSelector != null && !mTabModelSelector.isReparentingInProgress()
+                && tab != null) {
             tab.hide(TabHidingType.ACTIVITY_HIDDEN);
         }
     }