Steps to reproduce:

• Make sure you built Workbench with navigation modules
• Start Workbench
• Open "IGT Tracking Toolbox" view
• Select virtual tracker
• Click button "Add single tool"
• Click button "Finished"
• Click button "Connect"
• Open "IGT Navigation Tool Calibration" view
• On tab "Initialization", select "Virtual Tracker Tracking Source" and Tool
• Click button "Use as Tool to Calibrate"
• On tab "Tool Tip Calibration", change calibration method from "Manual" to "Single Reference Tool"

--> Workbench crashes