I had this problem and fixed it. In my example:
PreSave: Null Widget Tree - Expanded_UI_C /Game/UMG/Expanded_Button.WidgetArchetype
The problematic widget is Expanded UI. When I was able to right-click on it and go to reference viewer, I saw that there was an Expanded_Button reference with no value in it, which was causing the build to fail. I’m not even using this Expanded_Button anymore and it does not exist in my content browser, so my guess was I created it one point and when I deleted it, some traces of it were left behind.
I couldn’t delete this from the reference viewer unfortunately. I recreated the widget with the same name Expanded_Button and checked the reference viewer again for Expanded_UI widget, and I no longer saw it being referenced. I deleted the Expanded_Button after that and double checked once more from the reference viewer and it was now properly gone.
I assume once it found a proper matching name, it was able to resolve itself with the null value. Hope this helps.