[Kotlin] Fix required union accessors

src/idl_gen_kotlin.cpp

@@ -1138,8 +1138,13 @@ class KotlinGenerator : public BaseGenerator {
           case BASE_TYPE_UNION:
             GenerateFun(
                 writer, field_name, "obj: " + field_type, return_type, [&]() {
+                  const auto not_found =
+                      field.IsRequired()
+                          ? "throw AssertionError(\"No value for (required) "
+                            "field {{field_name}}\")"
+                          : "null";
                   writer += OffsetWrapperOneLine(
-                      offset_val, bbgetter + "(obj, o + bb_pos)", "null");
+                      offset_val, bbgetter + "(obj, o + bb_pos)", not_found);
                 });
             break;
           default:

tests/flatc/flatc_kotlin_tests.py

@@ -17,6 +17,20 @@
 
 class KotlinTests:
 
+  def RequiredUnion(self):
+    flatc(["--kotlin", "required_union.fbs"])
+
+    assert_file_and_contents(
+        "RequiredUnion.kt",
+        [
+            "fun payload(obj: Table) : Table {",
+            "else throw AssertionError(\"No value for (required) field "
+            "payload\")",
+        ],
+    )
+    assert_file_exists("Payload.kt").unlink()
+    assert_file_exists("PayloadUnion.kt").unlink()
+
   def EnumValAttributes(self):
     flatc(["--kotlin", "enum_val_attributes.fbs"])
 

tests/flatc/required_union.fbs

@@ -0,0 +1,11 @@
+table Payload {}
+
+union PayloadUnion {
+  Payload,
+}
+
+table RequiredUnion {
+  payload:PayloadUnion (required);
+}
+
+root_type RequiredUnion;