How do I write a test class for the following Method which makes callout to PowerBI?

Question

I need a test method to cover the following class:

Salesforce Embedded PowerBI

I tried to implement the HttpCalloutMock but I'm getting the following errors:

Attempt to de-reference a null object....

Class.PowerBiEmbedManager.EmbedTokenPlain.<init>: line 337, column 1

Class.PowerBiEmbedManager.getEmbedJSON: line 314, column 1

Class.PowerBiEmbedManager.getEmbeddingDataForReport: line 270, column 1

Class.PowerBiEmbedManager_TGN_TEST.getEmbeddingDataForReport_TGN_TEST: line 190, column 1