- Close all running instances of Visual Studio 2012
- start cmd.exe (as admin/elevated)
- cd /d %windir%\installer
- for /f %i in ('findstr /imc:"XAML UI Designer" *.msi') do (start /wait msiexec.exe -qb -x %i VSEXTUI=1)
Clear the prompts, allowing the uninstalls. - For the next two steps, order is important...
Go to Add/Remove Programs (AppWiz.cpl) and choose to repair all installations Visual Studio 2012 (Utimate, Pro, Premium, Win8 Express)
then...
Go to Add/Remove Programs (AppWiz.cpl) and choose to repair the Windows Phone 8 SDK
Please let us know if this resolves the problem for you. Again, apologies for the
inconvenience.
sources:http://social.msdn.microsoft.com/Forums/wpapps/en-US/7398ad5e-c2fc-40a8-85e7-37e6a6cca834/wp8-sdk-designer-crashes-vs-nullreferenceexception