TagTree doesn't display tags when other documents are open

[JAP1st]

Hi, great extension for those wanting to do social research with open source tools.

I think there's an issue with the TagTree panel, which doesn't display tags (even if they are present) when the document containing them is opened after other documents that have no QDA tags in it (only normal comments). Here are the bug details:

System:
Linux Mint 19.2, LibreOffice 6.0.7.3, QDA-TagTree 0.2.3

Current behaviour:
QDA TagTree panel doesn't show tags when document containing them is opened after other documents. It doesn't occur when document is opened first when starting LibreOffice.

Expected behaviour:
QDA TagTree panel shows tags in current document when working with several documents or when other documents were already opened.

Steps to reproduce:

1. Start LibreOffice.
2. Open any document not containing QDA tags (but perhaps normal comments).
3. Open the document containing QDA tags.
4. Try to update the panel --> Nothing happens.

Thanks!