Found the problem. It had nothing to do with the XAML itself. The exception was thrown due to inheritance
.
This control acts as parent for multiple other documents. And if the method shown in the Click event of the parameter isn't present in the 'sub'-controls then an unhandled exception is thrown.
Example:
MainControl A
=> Has method ButtonProcess_Click
SubControl B inherits MainControl A
=> Has method ButtonProcess_Click
SubControl C inherits MainControl A
=> Doesn't Have method ButtonProcess_Click
An exception will be thrown in SubControl C but not in A or B.