Thanks for the contact. Alex could help me to solve this problem.
It seems like there was a problem with the initialisation by starting the build.
“The TobiiAPI is implemented using so called ‘lazy initialization’. This means that the API is not initialized until the first call to any of its functions. Because of this, the first time the functions are called they will return values that are invalid (IsValid is false)”
So he added the if function “if(gaze.IsValid)” and the builds works now fine.