Fix tests

lib/src/code_generator/objc_built_in_functions.dart

@@ -326,7 +326,7 @@ class $name implements ${w.ffiLibraryPrefix}.Finalizable {
   String toString() {
     final data = dataUsingEncoding_(
         0x94000100 /* NSUTF16LittleEndianStringEncoding */);
-    return data.bytes.cast<${w.ffiPkgLibraryPrefix}.Utf16>().toDartString(
+    return data!.bytes.cast<${w.ffiPkgLibraryPrefix}.Utf16>().toDartString(
         length: length);
   }
 ''');

lib/src/code_generator/objc_interface.dart

@@ -369,6 +369,9 @@ class $name extends ${superType?.name ?? '_ObjCWrapper'} {
 
   String _getConvertedType(Type type, Writer w, String enclosingClass) {
     if (type is ObjCInstanceType) return enclosingClass;
+    if (type is ObjCNullable && type.child is ObjCInstanceType) {
+      return '$enclosingClass?';
+    }
     return type.getDartType(w);
   }
 

lib/src/code_generator/objc_nullable.dart

@@ -11,12 +11,15 @@ class ObjCNullable extends Type {
   Type child;
 
   ObjCNullable(this.child) {
-    assert(child is ObjCInterface ||
-        child is ObjCBlock ||
-        child is ObjCObjectPointer ||
-        child is ObjCInstanceType);
+    assert(isSupported(child));
   }
 
+  static bool isSupported(Type type) =>
+      type is ObjCInterface ||
+      type is ObjCBlock ||
+      type is ObjCObjectPointer ||
+      type is ObjCInstanceType;
+
   @override
   void addDependencies(Set<Binding> dependencies) {
     child.addDependencies(dependencies);

lib/src/header_parser/type_extractor/extractor.dart

@@ -141,7 +141,9 @@ Type getCodeGenType(
       );
       final isNullable = clang.clang_Type_getNullability(cxtype) ==
           clang_types.CXTypeNullabilityKind.CXTypeNullability_Nullable;
-      return isNullable ? ObjCNullable(innerType) : innerType;
+      return isNullable && ObjCNullable.isSupported(innerType)
+          ? ObjCNullable(innerType)
+          : innerType;
     default:
       var typeSpellKey =
           clang.clang_getTypeSpelling(cxtype).toStringAndDispose();