Detect null mail instance

During test runs the MailAppProvider can be null, so detect that first.

Bug: 7998963 Running unit tests coverage throws NPE in multiple tests
             in Gmail2

Change-Id: If068c7f3d95c82078757277e0e10696f53243134
diff --git a/src/com/android/mail/ui/AbstractActivityController.java b/src/com/android/mail/ui/AbstractActivityController.java
index 557d8f9..c04321b 100644
--- a/src/com/android/mail/ui/AbstractActivityController.java
+++ b/src/com/android/mail/ui/AbstractActivityController.java
@@ -1499,8 +1499,11 @@
         mActivity.invalidateOptionsMenu();
         disableNotificationsOnAccountChange(mAccount);
         restartOptionalLoader(LOADER_ACCOUNT_UPDATE_CURSOR);
-        MailAppProvider.getInstance().setLastViewedAccount(mAccount.uri.toString());
-
+        // The Mail instance can be null during test runs.
+        final MailAppProvider instance = MailAppProvider.getInstance();
+        if (instance != null) {
+            instance.setLastViewedAccount(mAccount.uri.toString());
+        }
         if (account.settings == null) {
             LogUtils.w(LOG_TAG, new Error(), "AAC ignoring account with null settings.");
             return;