How to prevent 'SaveAssetAs' when choosing img in UMG

On several occasions throughout 4.13 - 4.15 I’ve encountered the strange UMG behaviour that when selecting an image, I get the popup to ‘save asset as’ instead. Sometimes I can bypass this by restarting UE4, sometimes I must make a new widget.
What causes this behaviour and how can I prevent it?

I had the same problem as you. For some reason, sometimes when selecting an image, it brings up the “create new asset” window; but then instantly clicks on whatever asset is highlighted. To stop it from doing this; simply hold the select image button so it doesn’t auto click to create a new asset, and then select your image.