This actually happens if you played before the update but started a new game after updating.
You can try to load an old save once (this will "reset" the game) and then continue playing your new save, or delete everything from this folder
C:\Users\*username*\AppData\LocalLow\Glitch Pitch\Idol Manager\data\cache\portraits
Another option is to open your new save file with a text editor and replace "version":"Beta #11" with "version":"Beta #10"