I was 10.15.4 but reverted due to crappy scrolling changes so 10.15.3
All worked perfectly for me right until BTT restarted to install the update.
Yes icon selector works perfectly and they show
Then maybe for now search the icon selector for brightness (the ones in the batch section) and volume (the ones in the ionicons section) are the default icons.
I'll check whether I can reproduce this on my other machine.
I had a look but I couldn't find any icon that matched the default ones (They looked like the oem apple icons) but I will go through them all again and see.
Ah wait you are right the default ones are not in there.
That was my reason for using the widget over buttons. The icons look much better (Also fed into my request to extract all oem icons and provide them for use in the icons section but thats another topic)
Someone did that: [v3 Update] Native Apple Touch Bar Icon Pack for BTT
I can't include them in BTT for legal reasons.
The issue is very weird though. It seems like the system doesn't find the standard icons? How did you go about downgrading to 10.15.3? Do the icons still show correctly in the system Touch Bar?
You can also downgrade to any previous BTT version via https://folivora.ai/releases but I'm not sure whether it's really the alpha that causes the issue
Ah I missed that.
I used a time machine backup to restore but that was a week ago at least. All apple touchbar stuff is perfect and icons show fine. How/ where are the icons used for the widgets retrieved from?
If no custom icon has been assigned, they are referenced by their name and then the system frameworks handle showing them (e.g. NSTouchBarAudioOutputVolumeLowTemplate ),
So the icons are still there, apple can access them but BTT now can't. Was anything related to that changed in the Alpha?
I don't think so (and they are showing fine here), that's why I'm confused 
Maybe go to Help => Export Diagnostic Debug Information and send that to andreas@folivora.ai, this would allow me to reproduce your exact setup.
I will send that right away. Yes it is quite strange and you would expect it to happen on all systems if a path or similar was broken with the update, but software is software....
Thanks!
Unfortunately even with your exact setup it seems to show fine ;-(
I'm a bit out of ideas right now, let me do some tests.
Thanks, I will remove and reinstall an older version and use a backup I have from before I upgraded macos and report back
BTT automatically keeps backups for the last few versions in ~/Library/Application Support/BetterTouchTool, just downgrading to the last version you had installed should go back to that backup. (the files in that folder can tell you which version that was exactly)
Are there any hidden files or caches I should delete to ensure I'm getting a true "re install"?
Ok I completely deleted BTT and restarted and then reinstalled a version from beginning of April. Everything started working fine. I got the pop up for a new version is available, reinstalled the alpha and the icons are gone again!
very weird. The new versions are built with the new Xcode, maybe there is some strange incompatibility with 10.15.3. I’ll have access to a 10.15.3 machine tomorrow and do some tests!
I have an xcode update available in app store 
that shouldn’t be relevant, but there have been various cases in the past were new xcode versions broke stuff on „older“ macOS versions. It’s only relevant for the build machine though
I will install it now just to see, its an unusual problem