I don't know how to fix this NullPointerException

Question

So what I'm trying to do is to make a Dialog that shows when clicking a certain Button . This is the code:

final Dialog dialog = new Dialog(this, android.R.style.Theme_Translucent);
dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
dialog.setContentView(R.layout.alert_start);

            ImageButton ok = (ImageButton)findViewById(R.id.button_ok);
            ok.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
                    SharedPreferences settings = getSharedPreferences(CheckpointsActivity.PREFS_NAME, 0);
                    SharedPreferences.Editor editor = settings.edit();
                    editor.putInt(CheckpointsActivity.PREFS_CURRENT_CHECKPOINT, -1);
                    editor.putInt(CheckpointsActivity.PREFS_CURRENT_HINT, 1);
                    editor.putBoolean(CheckpointsActivity.PREFS_TIMER_IS_PAUSED, false);
                    editor.commit();

                    Intent intent = new Intent(PlaceItemActivity.this, CheckpointsActivity.class);
                    intent.putExtra(MainActivity.INTENT_PLACE_ITEM_ID, placeItem.id);
                    //intent.setClass(this, CheckpointsActivity.class);
                    startActivity(intent);
                }
            });

            ImageButton cancel = (ImageButton)findViewById(R.id.button_negative);
            cancel.setOnClickListener(new OnClickListener() {

                @Override
                public void onClick(View v) {
                dialog.dismiss();
            }
    });
    dialog.show();

As soon as I click the Button , app crashes. This is the logcat file:

07-24 20:45:00.261: E/AndroidRuntime(17139): FATAL EXCEPTION: main
07-24 20:45:00.261: E/AndroidRuntime(17139): java.lang.NullPointerException
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.view.ViewGroup.addViewInner(ViewGroup.java:3338)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.view.ViewGroup.addView(ViewGroup.java:3210)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.view.ViewGroup.addView(ViewGroup.java:3186)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:289)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at com.android.internal.policy.impl.PhoneWindow.setContentView(PhoneWindow.java:279)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.app.Dialog.setContentView(Dialog.java:482)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at si.dvanadva.evanturist.PlaceItemActivity.onClick(PlaceItemActivity.java:673)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.view.View.performClick(View.java:4204)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.view.View$PerformClick.run(View.java:17355)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.os.Handler.handleCallback(Handler.java:725)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.os.Handler.dispatchMessage(Handler.java:92)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.os.Looper.loop(Looper.java:137)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at android.app.ActivityThread.main(ActivityThread.java:5041)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at java.lang.reflect.Method.invokeNative(Native Method)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at java.lang.reflect.Method.invoke(Method.java:511)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
07-24 20:45:00.261: E/AndroidRuntime(17139):    at dalvik.system.NativeStart.main(Native Method)

The logcat says that the problem is in line 673. This is the line 673:

dialog.setContentView(R.layout.alert_start);

I really have no idea how to approach fixing this. I've had nullpointerexceptions before but never with Dialogs . What could be wrong?

alert_start xml code:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:id="@+id/alert_start"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical"
android:background="@color/dim"
android:padding="30dp"
 >

<ImageButton 
    android:id="@+id/button_ok"
    android:layout_width="58dp"
    android:layout_height="58dp"
    android:layout_alignParentRight="true"
    android:layout_centerVertical="true"
    android:background="@drawable/btn_forward"
    android:scaleType="fitXY"

     />

<ImageButton 
    android:id="@+id/button_negative"
    android:layout_width="58dp"
    android:layout_height="58dp"
    android:layout_alignParentLeft="true"
    android:layout_centerVertical="true"
    android:background="@drawable/btn_exit"
    android:scaleType="fitXY"

     />

<TextView
    android:id="@+id/textView2"
    android:layout_width="wrap_content"
    android:layout_height="50dp"
    android:layout_centerVertical="true"
    android:layout_toRightOf="@+id/button_negative"
    android:layout_toLeftOf="@+id/button_ok"
    android:layout_margin="5dp"

    android:text="@string/alert_start"
    android:gravity="center_horizontal|center_vertical"
    android:textAppearance="?android:attr/textAppearanceSmall"
    android:textColor="@color/black"
    android:background="@drawable/background_rounded_corners" />

Line 676:

ok.setOnClickListener(new OnClickListener() {

Answer 1

I believe your problem is with Context . Change

final Dialog dialog = new Dialog(this, android.R.style.Theme_Translucent);

to

final Dialog dialog = new Dialog(YourActivityName.this, android.R.style.Theme_Translucent);

The way you have it, this is referring to the onClickListener and not the Activity Context . This is your original problem.

Your next problem is, as MH said in a comment, you are trying to get the ok Button from the Activity layout but you need to get it from the Dialog layout so it is returning null .

Update

Try

ImageButton ok = (ImageButton) dialog.findViewById(R.id.button_ok);