[fix] handle invalid date in formatter. Fixes frappe/erpnext#4468

frappe/public/js/frappe/form/formatters.js

@@ -85,7 +85,15 @@ frappe.form.formatters = {
 		}
 	},
 	Date: function(value) {
-		return value ? dateutil.str_to_user(value) : "";
+		if (value) {
+			value = dateutil.str_to_user(value);
+			// handle invalid date
+			if (value==="Invalid date") {
+				value = null;
+			}
+		}
+
+		return value || "";
 	},
 	Datetime: function(value) {
 		return value ? dateutil.str_to_user(dateutil.convert_to_user_tz(value)) : "";