Merge pull request #20891 from hashicorp/b-helper-schema-setSet-panic

helper/schema: Prevent setSet() panic with typed nil

helper/schema/field_writer_map.go

@@ -341,6 +341,11 @@ func (w *MapFieldWriter) setSet(
 	// problems when the old data isn't wiped first.
 	w.clearTree(addr)
 
+	if value.(*Set) == nil {
+		w.result[k+".#"] = "0"
+		return nil
+	}
+
 	for code, elem := range value.(*Set).m {
 		if err := w.set(append(addrCopy, code), elem); err != nil {
 			return err

helper/schema/field_writer_map_test.go

@@ -203,6 +203,15 @@ func TestMapFieldWriter(t *testing.T) {
 			},
 		},
 
+		"set typed nil": {
+			[]string{"set"},
+			func() *Set { return nil }(),
+			false,
+			map[string]string{
+				"set.#": "0",
+			},
+		},
+
 		"set resource": {
 			[]string{"setDeep"},
 			[]interface{}{