Conference Paper/Poster version of Bachelor Thesis - should I cite the thesis?
Yes, you should always cite your own work and failing to do so can be called a violation of norms. This (self plagiarism) is, I think, a relatively recent concern, but it is something that everyone should give thought to in their writing. 

If at all possible, provide a new version of the publication, in which you make the correction before it is published and do something appropriate in the poster as well. 

Not everyone will complain and some will consider it a minor violation by a novice, but some might judge you harshly. Be aware of that and defend yourself against it. When in doubt, cite it. 

The purpose of avoiding plagiarism is a bit different than that of avoiding self plagiarism. Ordinary plagiarism is a violation because it represents someone else's work as your own. Self plagiarism doesn't do that, of course, so the purpose of avoiding it is different. 

When you present something that is derived from earlier, visible, work, and you cut and paste, you are likely to cut, past, and edit a bit. This can subtly change the meaning of things in a way that may not be obvious to a reader. Moreover, the original work appeared in a context including various references and other words that you did not cut and paste. This context is invisible to the reader of the new work. 

Therefore, when you use and update your old work you cite it in the new work. This permits a reader of the new work to find and analyze the context, just as they would if you were citing someone else's work.