Try setting a debugger breakpoint before where it crashes on you when your game gets to that point it'll stop and allow you to use the Evaluate & Modify tool. Go to Run -> Evaluate / Modify... and the tool will come up. In the Expression textbox you will want to put the name of your declared object in here.

It should either tell you it's null for sure or give you a list of all it's contents. If it is null then you may be referencing it improperly or freeing unknowingly.

What version of Lazarus are you using and on what OS? (I'm guessing Windows most likely?)