I am 3/4 of the way through the youtube video "Godot 3.1: Creating a Simple 3D Game: Part 5". I have just finished adding the blocks to the floor. Now nothing is displayed in the 3D window. I can play the scene and it looks right.
From the image you can see that the play scene looks OK but the 3D window in the background is empty.
I am pretty new to Godot so I certainly might have done something to cause this. I just need a pointer on how to undo whatever I did.
It is version 3.1.1 and I am running it on Mint Linux 19 - 64bit.