It is possible to create a custom field with type "TEXTBOX_SHORT" or "TEXTBOX_LONG" without specifying "textField" attribute. In this case when we try to add custom field we will get an NPE.
POST /custom-fields with following body passes validation and successfully creates custom text field.
This issue might also happen for type "SINGLE_CHECKBOX" and "checkboxField" attribute.
Above request should return an error if "textField" attribute is missing.