Call to a member function getFileSetID() on null

Hello,
When updating the website, I removed a block from stacks and broke the website. These are the steps I followed:

Step 1: I went to Stacks and Blocks> Stacks (image below)
Step 1b: clicked on the assistance module and clicked on ‘edit block’ (image below)
Step 2: opened the dropdown menu (image below)
Step 3: selected none (image below)

And that’s when I got the error

I can not access the page anymore (Call to a member function getFileSetID() on null), I need a solution how to fix this…

Cheers David