PHP error from message block

In blocks/messages/block_messages.php, this line of code:

$this->content->text = '';

is throwing this error:

PHP Warning:  Creating default object from empty value in blocks/messages/block_messages.php on line 34

It was introduced as part of commit b58961ac. I am uncertain how to fix this, else I would provide a patch for it.

To reproduce:

• disable messaging
• add messaging block on a page