Conversation-Block in Global Area does not work

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I'm builiding a blog page. An on the footer of any page I wanted to place the conversations block in a global stack - so it would appear on every page.

If i try to add a comment then there is a message over the textfield of the commenting section: "unexpected error occured"

In my console there is this error:
POSThttp://host-3:8888/frontiers82/index.php/tools/required/conversatio... 500 (Internal Server Error)

In the protocol section I found this entry:
Exception Occurred: /..../concrete/tools/conversations/add_message.php:14 Call to a member function getController() on null (0)

If I place that block just on the page out from the global stack, everything works as expected.

Status: New

concrete5 Environment Information

C5 8.2.1
PHP 7.0.20
Chrome 60.0.3112.101 / Mac

Browser User-Agent String

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_11_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.101 Safari/537.36