Skip properties from Java classes for which the getter type is unknown

thus avoiding NPE when auto-generating external serializers.

#KT-55681 Fixed

plugins/kotlinx-serialization/kotlinx-serialization.backend/src/org/jetbrains/kotlinx/serialization/compiler/backend/ir/IrSerializableProperties.kt

@@ -74,7 +74,7 @@ internal fun serializablePropertiesForIrBackend(
         if (irClass.isInternalSerializable) !it.annotations.hasAnnotation(SerializationAnnotations.serialTransientFqName)
         else !DescriptorVisibilities.isPrivate(it.visibility) && ((it.isVar && !it.annotations.hasAnnotation(SerializationAnnotations.serialTransientFqName)) || primaryParamsAsProps.contains(
             it
-        ))
+        )) && it.getter?.returnType != null // For some reason, some properties from Java (like java.net.URL.hostAddress) do not have getter. Let's ignore them, as they never have worked properly in K1 either.
 
     val (primaryCtorSerializableProps, bodySerializableProps) = properties
         .asSequence()

plugins/kotlinx-serialization/testData/boxIr/externalSerialierJava.kt

@@ -0,0 +1,28 @@
+// TARGET_BACKEND: JVM_IR
+
+// WITH_STDLIB
+
+import kotlinx.serialization.*
+import kotlinx.serialization.descriptors.*
+import kotlinx.serialization.encoding.*
+import java.net.URL
+
+
+// @Serializer should do nothing if all methods are overriden
+@Serializer(forClass = URL::class)
+object URLSerializer : KSerializer<URL> {
+    override val descriptor: SerialDescriptor = PrimitiveSerialDescriptor("java.net.URL", PrimitiveKind.STRING)
+
+    override fun serialize(encoder: Encoder, value: URL) {
+        TODO()
+    }
+
+    override fun deserialize(decoder: Decoder): URL {
+        TODO()
+    }
+}
+
+fun box(): String {
+    if (URLSerializer.descriptor.toString() != "PrimitiveDescriptor(java.net.URL)") return URLSerializer.descriptor.toString()
+    return "OK"
+}

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationFirBlackBoxTestGenerated.java

@@ -57,6 +57,12 @@ public class SerializationFirBlackBoxTestGenerated extends AbstractSerialization
             runTest("plugins/kotlinx-serialization/testData/boxIr/enumsAreCached.kt");
         }
 
+        @Test
+        @TestMetadata("externalSerialierJava.kt")
+        public void testExternalSerialierJava() throws Exception {
+            runTest("plugins/kotlinx-serialization/testData/boxIr/externalSerialierJava.kt");
+        }
+
         @Test
         @TestMetadata("genericBaseClassMultiple.kt")
         public void testGenericBaseClassMultiple() throws Exception {

plugins/kotlinx-serialization/tests-gen/org/jetbrains/kotlinx/serialization/runners/SerializationIrBoxTestGenerated.java

@@ -55,6 +55,12 @@ public class SerializationIrBoxTestGenerated extends AbstractSerializationIrBoxT
         runTest("plugins/kotlinx-serialization/testData/boxIr/enumsAreCached.kt");
     }
 
+    @Test
+    @TestMetadata("externalSerialierJava.kt")
+    public void testExternalSerialierJava() throws Exception {
+        runTest("plugins/kotlinx-serialization/testData/boxIr/externalSerialierJava.kt");
+    }
+
     @Test
     @TestMetadata("genericBaseClassMultiple.kt")
     public void testGenericBaseClassMultiple() throws Exception {