Broken weld on handlebar mount - trouble removing plastic frame cover

Hi everyone,
the other day, while I was adjusting the handlebar height, the weld that secures the handlebar broke.
I’m now trying to disassemble everything so that I can have it welded again, but I’m having some trouble removing the glossy plastic piece that connects the two parts of the frame.
Do you have any suggestions on how to take it apart? Thank you!