Apr 6, 2018 by M. Zemmy Indrapatih
If you got error notification like seen on image above when opening MS Visual Studio after upgrade to 15.5.x, you can follow the following step to fix it.
1. Launch Visual Studio
2. Open menu File -> New -> Project -> Xamarin App (either iOS or Android).
3. After project loaded completely, click the Toolbox. (At this point, VS would have its toolbox cache properly.
4. Close Visual Studio and then launch again. The error should be vanished now.