Guard against null tree more in TreeViewer.

E.g., getScaledTreeSize refs treeLayout, which is null if no tree.

Also add a way like getTree so we can test before invoking viewer methods.

Locate TreeViewer and inspect getScaledTreeSize along with the other viewer methods that may be called when no tree exists. Check how a null tree is represented, then add coverage showing the viewer can be checked before methods are invoked and handles the no-tree case safely.

Written by the indexing model from the issue text.