Android : Error OnClick Button in Custom Alert Dialog Android

Hey i want to add Google+ Sign In button in Alert Dialog. So I need to make custom alert dialog right. I create it, but somehow it cant receive OnClick action. I search through web and stackoverflow but somehow i still cant solve this problem. This is my Alert Dialog :

AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);

LayoutInflater layoutInflater = MainActivity.this.getLayoutInflater();
            builder.setView(layoutInflater.inflate(R.layout.dialog_signin, null))
                    .setTitle("Create Post")
                    .setPositiveButton("Sign In", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {
                            if (!mGoogleApiClient.isConnecting()) {
                                mSignInClicked = true;
                                resolveSignInError();
                            }
                        }
                    })
                    .setNegativeButton("Cancel", new DialogInterface.OnClickListener() {
                        @Override
                        public void onClick(DialogInterface dialog, int which) {

                        }
                    });

AlertDialog dialog = builder.create();

dialog.findViewById(R.id.sign_in_button).setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if (!mGoogleApiClient.isConnecting()) {
                    mSignInClicked = true;
                    resolveSignInError();
                }
            }
        });

dialog.show();

Please help me :) Thanks very much