Error when Adding an Event from Front End to Month Display with only one Category

I suspect the following might be a bug, alternatively let me know if I've misunderstood something.
I have a calendar displayed on the front end using a menu item to 'View by Month'. In the component options of the Menus:Edit Item I have selected only one JEvent category.
When this calendar is viewed by a user with sufficient permissions to add an event from the front end, and the user attempts to add a new event, the single category is displayed but as page text, not in a dropdown or other type of box. (See attached screenshot.)
When the new event is saved the page I get an Error:0 message with the following information "Call to a member function rp_id() on null" (See attached screenshot)
Going back to the calendar display the event is not displayed.
On checking the backend events, the new event is listed but the category is blank. If the category is inserted the event will display successfully.
As a workaround I discovered that adding 2 categories to the front end display caused the Add an Event dialogue to display an empty box for the category. Selecting the box enabled the single category to be selected and the event was successfully recorded and displayed.
I am new to this component so may have misunderstood something.