The bug

There is no error message when trying to generate from a nonexistent target pool in a jigsaw block.

Steps to reproduce

1. Summon a jigsaw block with all of the necessary data already inputted in it by using the command provided below.

   /setblock ~1 ~ ~1 minecraft:jigsaw{final_state:"minecraft:air",joint:"rollable",name:"minecraft:streets",pool:"minecraft:nonexistent_pool",target:"minecraft:streets"}

2. Open the jigsaw block, click on the "Generate" button, and notice how no error message is displayed.