Caused by calling getPassword() on a credentials that's not expecting it and not configured for this plugin:

https://github.com/jenkinsci/bitbucket-branch-source-plugin/blob/32cae450c88c13f89d3c27141fe5c9fac7331885/src/main/java/com/cloudbees/jenkins/plugins/bitbucket/api/credentials/BitbucketOAuthCredentialMatcher.java#L25