ringmainwindow: protect setTopAccount from null pointer exceptions

Change-Id: I2bc157c1656d9d4524bea8d8694909437da5cfb7
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>

src/ringmainwindow.cpp

@@ -426,7 +426,9 @@ on_account_changed(RingMainWindow* self)
             priv->cpp->enterAccountCreationWizard(true);
         } else {
             priv->cpp->leaveAccountCreationWizard();
-            priv->cpp->accountInfo_->accountModel->setTopAccount(accountId);
+            if (priv->cpp->accountInfo_) {
+                priv->cpp->accountInfo_->accountModel->setTopAccount(accountId);
+            }
             priv->cpp->onAccountSelectionChange(accountId);
             gtk_notebook_set_show_tabs(GTK_NOTEBOOK(priv->notebook_contacts),
                 priv->cpp->accountInfo_->contactModel->hasPendingRequests());