I managed to fix it. I added the whole doSearch() in the doBeforeRender() in a thread.
Which managed to show the exception in the logger. Which was a NullPointerException.
This was in the GetProductsBySearchTerms() at the line with "String clause = ", it couldn’t use the CategoryId correctly. So after I fixed that I could continue.
A lot of other fixes later the product search now works and is retrieved correctly.
Thanks for the help!