Add items to listbox using push button and drop down menu

I am creating a GUI using GUIDE in MATLAB2015. I have a drop down menu which the user selects a message to view, after that they click the Add push button to add the message name to a list box and display the data it contains in a table.

My problem is, if I want to add more than one message, instead of adding that message, it overwrites the previous one. Below is my current code:

addData = getappdata(handles.msgSel_menu, 'Data');
boxMsg = get(handles.msgSel_menu,'String');
boxMsgVal = get(handles.msgSel_menu,'Value');
set(handles.activeDataBox,'String',boxMsg{boxMsgVal});
set(handles.data_table, 'Data', addData);

Apologies if this has been done multiple times before, but as I'm relatively new to MATLAB I could do with a little explanation of any code that fixes my issue.

[Adam]

You just need to get the current string from the list box and concatenate your new string onto that before reassigning it to the listbox.

[Matt]

I've tried doing this but I keep getting the following error:

Cell contents assignment to a non-cell array object.

How do I avoid this?

[Adam]

Well, you haven't shown what you tried so it is impossible to say without guessing.

[Matt]

Sorry, I've now managed to add the different elements in, but they do not replace the initial string (e.g. 'Please Select...'), appear side by side and I am able to add more than one of the same message. Here is the code I currently have:

addData = getappdata(handles.msgSel_menu, 'Data');
boxMsg = get(handles.msgSel_menu,'String');
boxMsgVal = get(handles.msgSel_menu,'Value');
current_data = get(handles.activeDataBox, 'String');
new_data = [current_data, boxMsg{boxMsgVal}];
set(handles.activeDataBox,'String',new_data);
set(handles.data_table, 'Data', addData);
set(handles.activeDataBox, 'String', new_data);

[Adam]

Well, there isn't any magic to creating a cell array - either you replace elements or concatenate with them.

If you want special rules to remove a certain string or not allow duplicates you would just have to program the logic in for that yourself when doing the concatenation.