Robustify cloning after reloading XML

[rbouckaert]

Currently, this will result in an error:

• import Primates.nex into Standard template
• link trees
• save file
• close and reopen BEAUti and load saved file
• in clock panel, select ORC for first partition
• clone second partition from first partition

The last step results in an error message: Could not clone secondhalf to firtshalf Cannot invoke "beast.base.evolution.tree.Node.toString()" because "this.root" is null