travel-distance variables input is crashing gui

[anna-abelman]

when adding variables to var1, the gui crashes

[Paul-Carvalho]

@anna-abelman, I don't think this is a bug in the code, but user error when adding variables. The variable must be numeric - not character or factor class.

We should add a statement in the code to check variable class for travel-distance variables and gid varying variable and generate an error message if classes are invalid.