ERROR: Cannot read properties of null (reading 'x3ServiceClientId')

After installing my X3 instance, when I try to access the XTrem Explorer URL (http://localhost:8124/xtrem/explorer) I get the following error:

Cannot read properties of null (reading 'x3ServiceClientId')

You could see this error in two situations:

1.  You do not have a valid X3 session.  i.e. you must login to X3 before the XTrem Explorer will allow access

2.  "X3 Service" has not been selected in Administration, Administration, Endpoints, X3 Solution