Add a null check in an AutoCompleteTextView callback to prevent a crash.

It looks like the drop down list had already been taken down before the
callback fired so it caused a null pointer exception.  In this case just
do nothing instead.

Bug 1927858

core/java/android/widget/AutoCompleteTextView.java

@@ -1202,7 +1202,11 @@ public class AutoCompleteTextView extends EditText implements Filter.FilterListe
                         int position, long id) {
 
                     if (position != -1) {
-                        mDropDownList.mListSelectionHidden = false;
+                        DropDownListView dropDownList = mDropDownList;
+
+                        if (dropDownList != null) {
+                            dropDownList.mListSelectionHidden = false;
+                        }
                     }
                 }