Isuse with messagebundle.properties in SAP UI 5

I am fairly new to SAP UI 5 and am a little confused as to why I am not seeing the correct descriptive text for elements within the framework.

I am using the sap.m.TableSelectDialog in my application and have based my work on the example here:

http://ift.tt/2yIALMI

In the example you can see the use of the cancel button named 'Cancel'

However in my application the cancel button's name is set to 'MSGBOX_CANCEL'

I understand that the exact description is set in the messagebundle.properties file in a format like:

MSGBOX_CANCEL=Cancel

Could anyone point me in the right direction as to why this is not working for me?

I also receive the following error in the console:

The file is present though.

Cheers, James