"Use custom working directory" is greyed out in Edit Scheme > Run

I'm trying to use XCode to debug a simple toy C++ program built using a Makefile (set up project using 'External Build System').

Normally I'm able to debug and run, and use breakpoints within the editor. The way I did this was by going to Edit Scheme > Run > Use custom working directory, then checking the check box and specifying the directory of the executable. Except currently I'm not able to click in the text field or on the folder pop-up dialog at all to change this directory from blank.

Such that, when I hit the build and run button with the check box checked I get a popup that asks Would you like to Run anyway? : The working directory "(null)" for scheme "scheduler" doesn't exist. Except I can't change the blank text field at all to fix this...

I would add a screenshot of the options to clarify but I'm concerned about triggering moderation given this post I read about the forums [https://forums.developer.apple.com/thread/97547]