fix: Throw "Invalid Filter Value" error on getting TypeError

TypeError with message "'NoneType' object is not iterable" will mostly occur if a link field value does not exist.

frappe/core/page/dashboard/dashboard.py

@@ -3,7 +3,8 @@
 from __future__ import unicode_literals
 import json
 import frappe
-from frappe.utils import add_to_date
+from frappe import _
+from frappe.utils import add_to_date, get_link_to_form
 
 
 def cache_source(function):
@@ -29,7 +30,21 @@ def cache_source(function):
 	return wrapper
 
 def generate_and_cache_results(chart, chart_name, function, cache_key):
-	results = function(chart_name = chart_name)
+	try:
+		results = function(chart_name = chart_name)
+	except TypeError as e:
+		if e.message == "'NoneType' object is not iterable":
+			# Probably because of invalid link filter
+			#
+			# Note: Do not try to find the right way of doing this because
+			# it results in an inelegant & inefficient solution
+			# ref: https://github.com/frappe/frappe/pull/9403
+			frappe.throw(_('Please check the filter values set for Dashboard Chart: {}').format(
+				get_link_to_form(chart.doctype, chart.name)), title=_('Invalid Filter Value'))
+			return
+		else:
+			raise
+
 	frappe.cache().set_value(cache_key, json.dumps(results, default=str))
 	frappe.db.set_value("Dashboard Chart", chart_name, "last_synced_on", frappe.utils.now(), update_modified = False)
 	return results