[TabLayoutMediator] Prevent crash autoRefresh disabled mediator

Resolves #877

GIT_ORIGIN_REV_ID=db2949c4ed5358867e83b3e1cabad00e574ba5ba
PiperOrigin-RevId: 288822885

(cherry picked from commit 07931b6)

Author: ryohji

lib/java/com/google/android/material/tabs/TabLayoutMediator.java

@@ -136,12 +136,12 @@ public void attach() {
    * called before {@link #attach()} when a ViewPager2's adapter is changed.
    */
   public void detach() {
-    if (adapter != null) {
+    if (autoRefresh && adapter != null) {
       adapter.unregisterAdapterDataObserver(pagerAdapterObserver);
+      pagerAdapterObserver = null;
     }
     tabLayout.removeOnTabSelectedListener(onTabSelectedListener);
     viewPager.unregisterOnPageChangeCallback(onPageChangeCallback);
-    pagerAdapterObserver = null;
     onTabSelectedListener = null;
     onPageChangeCallback = null;
     adapter = null;