on delete contact update issue (#14770)

* on delete contact update isuue

remove contact from issue if the deleted contact exists

* moved issue update function to issue.py

* Update formatting and comment
diff --git a/erpnext/hooks.py b/erpnext/hooks.py
index b461e16..8d09f29 100644
--- a/erpnext/hooks.py
+++ b/erpnext/hooks.py
@@ -210,6 +210,9 @@
 	},
 	('Sales Invoice', 'Purchase Invoice', 'Delivery Note'): {
 		'validate': 'erpnext.regional.india.utils.set_place_of_supply'
+	},
+	"Contact":{
+		"on_trash": "erpnext.support.doctype.issue.issue.update_issue"
 	}
 }
 
diff --git a/erpnext/support/doctype/issue/issue.py b/erpnext/support/doctype/issue/issue.py
index dfcc2a8..ef54b20 100644
--- a/erpnext/support/doctype/issue/issue.py
+++ b/erpnext/support/doctype/issue/issue.py
@@ -130,3 +130,8 @@
 
 def has_website_permission(doc, ptype, user, verbose=False):
 	return doc.raised_by==user
+
+
+def update_issue(contact, method):
+	"""Called when Contact is deleted"""
+	frappe.db.sql("""UPDATE `tabIssue` set contact='' where contact=%s""", contact.name)