fix: dont assume issingle exists (#18236) (#18252)

(cherry picked from commit 0e0e7f276c)

Co-authored-by: Ankush Menat <ankush@frappe.io>

frappe/model/base_document.py

@@ -744,7 +744,7 @@ class BaseDocument:
 						# don't cache if fetching other values too
 						values = frappe.db.get_value(doctype, docname, values_to_fetch, as_dict=True)
 
-				if frappe.get_meta(doctype).issingle:
+				if getattr(frappe.get_meta(doctype), "issingle", 0):
 					values.name = doctype
 
 				if frappe.get_meta(doctype).get("is_virtual"):
@@ -884,7 +884,7 @@ class BaseDocument:
 		if frappe.flags.in_install:
 			return
 
-		if self.meta.issingle:
+		if getattr(self.meta, "issingle", 0):
 			# single doctype value type is mediumtext
 			return