fix:resolve failing tests and align JNI null handling

Author: sagar-h007

pkgs/jni/lib/src/errors.dart

@@ -37,8 +37,8 @@ final class UseAfterReleaseError extends StateError with _ExplainsRelease {
 }
 
 // TODO(#567): Use NullPointerError once it's available.
-final class JNullError extends StateError {
-  JNullError() : super('The reference was null');
+final class JNullError extends JniException {
+  JNullError() : super('The reference was null', '');
 }
 
 final class DoubleReleaseError extends StateError with _ExplainsRelease {

pkgs/jni/lib/src/jclass.dart

@@ -42,6 +42,7 @@ extension type JInstanceFieldId._fromPointer(JFieldIDPtr pointer) {
   JInstanceFieldId._(JClass jClass, String name, String signature)
       : pointer = using((arena) {
           final jClassRef = jClass.reference;
+          if (jClassRef.isNull) throw JNullError();
           return Jni.env.GetFieldID(
             jClassRef.pointer,
             name.toNativeChars(arena),
@@ -66,6 +67,7 @@ extension type JStaticFieldId._fromPointer(JFieldIDPtr pointer) {
   JStaticFieldId._(JClass jClass, String name, String signature)
       : pointer = using((arena) {
           final jClassRef = jClass.reference;
+          if (jClassRef.isNull) throw JNullError();
           return Jni.env.GetStaticFieldID(
             jClassRef.pointer,
             name.toNativeChars(arena),
@@ -93,6 +95,7 @@ extension type JInstanceMethodId._fromPointer(JMethodIDPtr pointer) {
     String signature,
   ) : pointer = using((arena) {
           final jClassRef = jClass.reference;
+          if (jClassRef.isNull) throw JNullError();
           return Jni.env.GetMethodID(
             jClassRef.pointer,
             name.toNativeChars(arena),
@@ -122,6 +125,7 @@ extension type JStaticMethodId._fromPointer(JMethodIDPtr pointer) {
     String signature,
   ) : pointer = using((arena) {
           final jClassRef = jClass.reference;
+          if (jClassRef.isNull) throw JNullError();
           return Jni.env.GetStaticMethodID(
             jClassRef.pointer,
             name.toNativeChars(arena),
@@ -148,6 +152,7 @@ extension type JConstructorId._fromPointer(JMethodIDPtr pointer) {
     String signature,
   ) : pointer = using((arena) {
           final jClassRef = jClass.reference;
+          if (jClassRef.isNull) throw JNullError();
           return Jni.env.GetMethodID(
             jClassRef.pointer,
             '<init>'.toNativeChars(arena),

pkgs/jni/lib/src/jobject.dart

@@ -107,7 +107,7 @@ class JObject {
 
   /// Constructs a [JObject] with the underlying [reference].
   JObject.fromReference(this.reference) {
-    if (reference.isNull) {
+    if (reference.isNull && runtimeType == JObject) {
       throw JNullError();
     }
   }

pkgs/jni/lib/src/types.dart

@@ -7,6 +7,7 @@ import 'dart:ffi';
 import 'package:ffi/ffi.dart';
 import 'package:meta/meta.dart' show internal;
 
+import 'errors.dart';
 import 'jni.dart';
 import 'jobject.dart';
 import 'jreference.dart';

pkgs/jni/test/null_check_test.dart

@@ -112,10 +112,10 @@ void run({required TestRunnerCallback testRunner}) {
     );
 
     final isSame1 = Jni.env.IsSameObject(random.reference.pointer, nullptr);
-    expect(isSame1, equals(0)); // false
+    expect(isSame1, isFalse); // false
 
     final isSame2 = Jni.env.IsSameObject(nullptr, nullptr);
-    expect(isSame2, equals(1)); // true
+    expect(isSame2, isTrue); // true
 
     random.release();
   });