Playing in "new editor window" reports errors, selected viewport and simulate do not

I have been testing my game in-editor with the “Selected Viewport” option, and everything works fine. But when I run it in a new editor window, I get thousands of “Accessed none” errors that all seemingly reference the wrong node. The blueprint where these errors are coming form works correctly however, so I don’t understand why I get any errors to begin with. I disconnected everything, and found that if either of two functions are called, I get the errors. Depending on which function is connected, it’s either a “Slot As Canvas” or “Get Player Pawn” (or both if I call the two functions) node that gives me the error, but they seem to work properly though, the BP does what I expect it to do. Even if I disconnect the node that is supposedly causing the issue, it still says that’s where the error occurs.

I’m attaching a screenshot from the simpler functions that just updates the size and position of a couple UMG Widget borders every time it is called. I just use a sequence to update them, the only differences are the input positions and the slot variable. I call set size and set position for five different borders, and that’s all the function does, but it gives me the Slot As Canvas error in an unrelated node outside of the function.

Why do I only get errors in the new editor window mode, and not selected viewport or simulate?

Thanks for any help.