I discovered a workaround (not a fix!) with some Trial & Error.
1) Open up ~/.kde/share/config/plasma-desktop-appletsrc in a text editor
2) Search for Items= (for me there was only one entry with this prefix)
2a) If you have a APPNAME= line underitems for the affected application, delete it. For me I had a 8k line of base64 encoded text starting chrome=
3) The items line is a CSV string. Look for the file:/// reference to the broken launcher. For me it was google chrome and I had a reference to: file:///opt/google/chrome/chrome?wmClass=google-chrome-stable
4) Navigate to /usr/share/applications/ and find the .desktop file for the application you're trying to run. (If it's not there also look in /usr/share/applications/kde4/ )
5) Replace the entry in the Items=line with the path to the .desktop file but leave ?wmClass and everything after it.
So for me:
file:///opt/google/chrome/chrome?wmClass=google-chrome-stable
became
file:///usr/share/applications/google-chrome.desktop?wmClass=google-chrome-stable
6) Finally, open konsole and restart plasma with the commands:
- kquitapp plasma-desktop
- plasma-desktop
edit: I should also say, if there is no .desktop file there, open an existing one in a text editor and use it as a base to create a new one.