Permission error when importing with public admin

[tusmester]

When we try to import files to https://daily.test.sensenet.cloud in the name of the public admin user, we receive a permission error:

Invalid property access attempt on a See-only node: /Root

This is caused by the Folder.IsPreviewEnabled property that tries to access the parent.

public virtual bool IsPreviewEnabled => Parent?.IsPreviewEnabled ?? false;

Check and solve this (probably with elevation) in one or both of the following places:

• Folder class
• Node class

Add property name info to the trace message above.

[kavics]

The merged modification does not fix the basic problem. See also: #1861