[Fix]Root element from tree cannot be deleted.

webnotes/utils/nestedset.py

@@ -32,7 +32,7 @@
 from __future__ import unicode_literals
 
 import webnotes, unittest
-from webnotes import msgprint
+from webnotes import msgprint, _
 from webnotes.model.bean import Bean
 from webnotes.model.doc import Document
 
@@ -325,5 +325,9 @@ class DocTypeNestedSet(object):
 		update_nsm(self)
 		
 	def on_trash(self):
-		self.doc.fields[self.nsm_parent_field] = ""
+		parent = self.doc.fields[self.nsm_parent_field]
+		if not parent:
+			msgprint(_("Root ") + self.doc.doctype + _(" cannot be deleted."), raise_exception=1)
+
+		parent = ""
 		update_nsm(self)