See this discussion and this SECURITY-144 exemption for context. SubversionSCM.descriptor() and SubversionWorkspaceSelector.syncWorkspaceFormatFromMaster must not call Jenkins.getInstanceOrNull or similar. Checking JenkinsJVM is the quick way to determine if code is running on the master or an agent, but really this check should not be necessary at all. Rather, all information about master-side configuration should be prepared on the master and included in the master → slave callable's constructor and serializable state (CheckOutTask for example). PerJobCredentialStoreTest.remoteBuild is a key test.
When fixed and released, the callable.conf entry can be deleted.