Error message when clicking "Cancel" in web services test client

Mahara 15.10+

When you are on the web services test client's last page and see the "wstoken" and you click "Cancel", you get the following error message instead of getting back to the start page for the test client:

Mahara: Invalid Parameter
A required parameter is missing or malformed

The 'wstoken' parameter is not alphanumeric only

Steps to reproduce:

1. Choose a protocol.
2. Choose an auth type.
3. Click the *Next* button.
4. Choose the service group that you want to test.
5. Click the *Next* button.
6. Select the function that you want to include in your test.
7. Click the *Next* button.
8. Instead of "Execute", click "Cancel".

Expected result: Be taken to the beginning of the web services test.
Actual result: Above error message.