#014124: better error handling for $tree->accept, if no root node is defined

If I call accept on a tree, which doesn't have a root node, I get:

Fatal error: Call to a member function accept() on a non-object in /var/checkouts/ezcomponents/trunk/Tree/src/tree.php on line 236

This should maybe handled with a more explanatory exception.

Operating System:
PHP Version: (please be specific, like '4.4.3' or '5.1.5')
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