Fix exception when no selection in HyperTreeList

If a HyperTreeList is created with only the root item present, and not
selected, an exception will be thrown if any key is pressed.
GetFirstVisibleItem() returns None, even though the root item is
present and visible. Only an issue if the TR_MULTIPLE flag is set.

wx/lib/agw/hypertreelist.py

@@ -2348,7 +2348,15 @@ class TreeListMainWindow(CustomTreeCtrl):
     def GetFirstVisibleItem(self):
         """ Returns the first visible item. """
 
-        return self.GetNextVisible(self.GetRootItem())
+        root = self.GetRootItem()
+        if not root:
+            return None
+
+        if not self.HasAGWFlag(TR_HIDE_ROOT):
+            if self.IsVisible(root):
+                return root
+            
+        return self.GetNextVisible(root)
 
 
     def GetPrevVisible(self, item):