Area scattering defaults to 0.01 spacing when input is invalid

If the user provides an invalid input for Area spacing (for example, using the hotkey 'R' to check the raytraced image), the spacing defaults to a value of 0.01m. Depending on the size of the Area scattering, this can freeze Twinmotion and potentially cause an OOM crash. 

1. Create a sizeable Area with one or more objects (ex. 2 TM library trees) and probability = 1. 
2. Change spacing value to invalid input (ex. 'R') and press enter.
3. Attempt to change the spacing value.

Result: Twinmotion freezes, crashes