[Typo3] Extension Kickstarter in TYPO3 3.8.0 - namimg error problem

[Helmut Menold]

Hello, i try to set up my extension (product data pooling) with the
kickstarter.
Step by step I followed the video Tutorial (Episode III) :
- I setup the general Info
- I created 2 tables one with the core data and the other with the
categories and I set the relation
- I created the frontend plugin
Everything works fine. The records of both tables can be created in the BE
(core data with selectbox for categories). After inserting the frontend
plugin as content element the FE shows the records ... good so far.

As Kasper did in the video I want to extend the plugin with a selectbox for
the categories. I extend the existing table tt_content with a field with
relation to the categories table.

But the selectbox does not appear in the content elements frontend plugin.
The information site of my extension displays: 

Naming errors: tables tt_content tt_content
 
 
Need some help. Thank you.  

regards
Helmut Menold