How to reproduce

• Start with a clean qtcreator, without any qt versions set or in the path.
• Create a new Qt4 Project: "Qt in Path" is selected as only target on the last pane in the wizard.
• Projects pane correctly complains about "Qt Version is invalid"
• Add your favorite Qt version in Options -> Qt4 (Toolchain is correctly shown here)
• Switch to new valid Qt version
• Tool Chain box is still empty/disabled
• "Make Step: make not found in the environment." is still shown

Closing & Reloading the project fixes this.