"join" formtool specifies that "checkbox" is a valid render type when it is not

The join formtool specifies in the metadata for ftRenderType that "checkbox" is a valid option (along with library and list). In fact, "radio" is the valid option, but this only works for ftType="uuid" not ftType="array".

"checkbox" should be a valid option for ftType="array" properties. "radio" should remain for ftType="uuid" properties.