Site import failing with error message "User should not be null" when the username is different between the source and destination instance for the importer

Summary

When performing a site import, if the user performing the import has a different username set on the source instance and on the destination instance, the import operation will fail with the error User should not be null.
The progress bar of the site import will reach 100% and starts removing the imported content.

Steps to Reproduce

1. Import a site backup on a new Confluence Cloud instance. Usernames must be different between the source instance and the destination instance for the user performing the import.

Expected Results

The import completes properly.

Actual Results

The import fails but still reaches 100%. The following error is thrown in Confluence logs:

	 msg:	 User should not be null	
    	 stack:	 java.lang.NullPointerException: User should not be null
	at java.util.Objects.requireNonNull(Objects.java:228)
	at com.atlassian.crowd.embedded.atlassianuser.EmbeddedCrowdGroupManager.getCrowdUser(EmbeddedCrowdGroupManager.java:181)
	at com.atlassian.crowd.embedded.atlassianuser.EmbeddedCrowdGroupManager.addMembership(EmbeddedCrowdGroupManager.java:137)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
...

Notes

This behaviour is often encountered as when creating a new Cloud instance to perform a site import, the default username is admin while the user performing the import is not always the original creator on the source instance. Therefore the usernames differ.

Workaround

Change your username to the one used in the source instance and perform the import once again.

EDIT:
As usernames are no longer changeable, if you encounter this issue, invite a new user that was a site administrator on your source instance with the same email address. This user should be able to perform the import.