Crash while trying to figure out how to make a multistory building

I was messing with a multistory building, and the crash occurred while I was trying to add a floor tile on the top floor while using the layers function. The exception message popped up twice -- it was both a NullReferenceException but I'm not sure if they were otherwise different.

```

Noble Fates Critical Error:

NullReferenceException: Object reference not set to an instance of an object

UIPropPreviewWindowContents.SetTitle (System.String title) (at <344d58956cfa41578e34351a88700f9a>:0)

PropManager.RefreshPreview () (at <344d58956cfa41578e34351a88700f9a>:0)

TilePropToolsSelectionMode.Update () (at <344d58956cfa41578e34351a88700f9a>:0)

PropManager.HandleMouse () (at <344d58956cfa41578e34351a88700f9a>:0)

SelectionManager.Tick (System.Single dt) (at <344d58956cfa41578e34351a88700f9a>:0)

ManagerBehavior.Update () (at <344d58956cfa41578e34351a88700f9a>:0)

```