Lets take the aegisalore that generates this error

Here is the best way to prevent that. Take your existing patch here
and replace with something like this

This is a versatile method. This uses patch batches by nesting everything in an extra array []
The value of this is that the failure of one batch won't cause another batch to fail. And failed "test" operations won't generate log errors either.

The first batch tests for the existence of that path. But the "inverse" : true reverses the condition - which means it is testing for the lack of that path. If that condition is satisfied, it then adds the path, with a blank array as the value. If some other mod already added the path, this will not do so.

The second part is what you already had. It adds the "aegisblock" to the end of the array. Only now there is certainly an array to add to!

In the event that you are trying to keep this mod working with another mod(that doesn't use this method of patching) - but you don't require that mod necessarily - it would be best if this mod loaded after the other. You can accomplish this in _metadata like so.

You use the actual mod's "name" string of course.

If you have any questions, I'd be glad to help.