Invalid editor window UnityEditor.FallbackEditorWindow UnityEditor.EditorApplicationLayout:SetPlaymodeLayout()

Best answer is cool, but you not need to reset all settings, for me it was solved just by changing layout and returning back to my own.

i was having a Similar problem and the solution seemed to be that i didnt have my layout set to anything. If you see [Layout] at the top right of your screen then thats probably why and u need to either set it to what ever custom layout youve made or a defualt layout but it shouldnt say layout regardless. if this doesnt work then you can reset to default but thats last resort because then you delete your custom layouts. @N1warhead

I didn't want to delete the layers and found another solution. It worked for me by closing the scene window (just right click the scene tab and click on close tab) and then reopen it again with

Window > General > Scene.

The other solutions commented here didn't work for me. Maybe this is the solution for you.

This is another really STUPID error, and shouldn't even exist. Unity seems to want you to pick an existing saved layout - either a factory preset, or one you've saved - and then never change it without updating the saved layout. The only problem is that during normal use, Unity will often forget which layout you're using. Maybe you accidentally moved a window border, or some other minor detail about the layout changes - then next time you start Unity, the selected layout will be blank, and you start getting errors. Totally unnecessary to even check for this, let alone pester users all the time with annoying errors about NOTHING.