Sometimes in Legacy\View\Provider\Block, legacy kernel fails to return eZObjectForwarder for block_view_gui. This happens most likely when there is a cache issue (e.g. context change when developing).
It results to a fatal error:
PHP Notice: Undefined index: block_view_gui in /Users/lolautruche/workspace/ezsystems/ezpublish-legacy/lib/eztemplate/classes/eztemplate.php on line 660
PHP Fatal error: Call to a member function process() on a non-object in /Users/lolautruche/workspace/ezsystems/ezpublish-kernel/eZ/Publish/Core/MVC/Legacy/View/Provider/Block.php on line 59
Potentially also occurs with Legacy\View\Provider\Content.