Integrate database with history.

Bug: 31623549

Note: This CL operates under the assumption that there is something
in the current expression field (handling for various states of
CalculatorDisplay in ag/1613446)

Change-Id: I70992067ddc9c5eec079f00604549727787e26fe
diff --git a/src/com/android/calculator2/AlertDialogFragment.java b/src/com/android/calculator2/AlertDialogFragment.java
index e098022..12840ae 100644
--- a/src/com/android/calculator2/AlertDialogFragment.java
+++ b/src/com/android/calculator2/AlertDialogFragment.java
@@ -20,6 +20,7 @@
 import android.app.AlertDialog;
 import android.app.Dialog;
 import android.app.DialogFragment;
+import android.app.FragmentManager;
 import android.content.DialogInterface;
 import android.os.Bundle;
 import android.support.annotation.Nullable;
@@ -78,6 +79,10 @@
      */
     public static void showMessageDialog(Activity activity, @Nullable CharSequence title,
             CharSequence message, @Nullable CharSequence positiveButtonLabel) {
+        final FragmentManager manager = activity.getFragmentManager();
+        if (manager == null || manager.isDestroyed()) {
+            return;
+        }
         final AlertDialogFragment dialogFragment = new AlertDialogFragment();
         final Bundle args = new Bundle();
         args.putCharSequence(KEY_MESSAGE, message);
@@ -87,7 +92,7 @@
         }
         args.putCharSequence(KEY_TITLE, title);
         dialogFragment.setArguments(args);
-        dialogFragment.show(activity.getFragmentManager(), null /* tag */);
+        dialogFragment.show(manager, null /* tag */);
     }
 
     public AlertDialogFragment() {