The 'Sample buttons and forms' window automatically pops up when I start InDesign. Indesign starts with the workspace 'Start'. I resetted my workspace and the window still was there. Then I removed the window, saved my workspace again and it seems to be solved.
I am not the only one with this problem:
https://www.reddit.com/r/indesign/comments/d1s9yf/sample_buttons_forms_window_automatically_pops_up/