I found out this problem - luckily in quick order.
For some reason, B4A 4.x corrupted my project.b4a file today - it had these two lines in it:
Build1=,com.omnisoft.ackbrowser
Build2=DEBUG,com.omnisoft.ackbrowser,ISME
As you can see the first parameter in "Build1" is missing, causing the initial problem. So I replaced both lines with the single default value:
Build1=Default,com.omnisoft.ackbrowser
But then I got another error:
"An error occurred.
Error loading file.
index was out of range. Must be non-negative and less then the size of the collection.
Parameter name: Index"
This seems to have been caused by my project.b4a.meta having these entries in it:
ModuleClosedNodes3=59
SelectedBuild=1
This second error appears to been caused by me reducing the number of builds in my project.b4a file from 2 to one (to fix the first problem), but the meta file had "SelectedBuild=1", which now doesn't exist, so I had to change it to "0" so its in sync with the change I made to the project.b4a file above.
I don't know what the "ModuleClosedNodes3=59" line does, but the version of my project that loaded OK from yesterday had a value if 58, so I also did this change to make sure this issue will be fixed.