To reproduce: Right-click on function name, select “display references”
this image displays: 
The text reads:
PHP.VisualStudio.Language.Nodes.Ast.GlobalCodeNode’ cannot be read
Parameter name:file
Not sure if I did something wrong to get to this state. It has just started happening.
OOPS - restart Visual Studio and it goes away! So maybe no problem. And I noticed I had tried to use "find all references" while I was debugging (top arrow "Web Server" debug)
I'll leave this post for the next guy. I'm happy.
Microsoft Visual Studio Enterprise 2019
Version 16.11.5