If you wish to bypass this error please follow the directions here:
Download: http://fixes.dungeondefenders2.com/No_Launcher.zip
- Extract the .zip
- Take both extracted files and place them directly into this folder: C:\Program Files (x86)\Steam\SteamApps\common\Dungeon Defenders 2\DunDef2\Binaries\Win32
- Double click the No_Launcher.bat that you copied over into the folder and the game should launch.