Please check the Model under test. In this case, PageModel.class - If any of the injects/references used in sling model is null in the context of test environment, then it will result in Model being null.
We need to set those references used in Model to aemContext and then use it to get the Model object.
Example: If a Model uses Externalizer service, then we need to set the Externalizer to aemContext first before instantiating the Model.
Please share the PageModel.class if it is fine to share for further debugging.