@blastframe For me, it works fine this way. Did you really "close and reopen" the document? Not "save", "close", "load"? Because if you just close the document, the data from the container is gone, which leads to your error message.
The Q regarding MyUniqueId vs. PLUGIN_ID is not relevant (probably typed in a hurry), you can use either. But the ID value should come from Maxon in the end, so you avoid collisions with existing IDs.