Prevent crash described in #34555.

Normally it wouldn't be possible to click on the "Connect" button if no node
is selected, because the button would be disabled.

However, double clicking on a node is also hooked up to the same signal
and double clicking is possible even if the "Connect" button is disabled.

This caused a crash described in #34555.

editor/connections_dialog.h

@@ -76,6 +76,7 @@ class ConnectDialog : public ConfirmationDialog {
 	void ok_pressed();
 	void _cancel_pressed();
 	void _tree_node_selected();
+	void _tree_item_activated();
 	void _add_bind();
 	void _remove_bind();
 	void _advanced_pressed();