Invalid Data error when trying to open a new level

An invalid data error is given when trying to save all levels.

1. Download the attached project
2. Open the project
3. Open the level "Untitled"
4. Click the build button
5. Open level "Test2" via File > Open level > Test2 > Open
6. Save all (this prompt will pop up after clicking the open button in the step above)

Result: An invalid data error occurs when trying to save all
Expected: The level would save without error