Android Dialog NoSuchMethodException error when using XML onClick

I'm new to Java and Android, and I'm working on my first test app.

I've progressed with it, but I'm blocked with a Dialog.

I show the dialog from the Activity like this:

//BuyActivity.java
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_shop);

    initialize_PR();
    display_PR();
    BuyDialog=new Dialog(this);
    BuyDialog.setContentView(R.layout.dialog_buy);

}
public void Action_ShowDialog_Buy(View view) {
    BuyDialog.show() ;
}

And the dialog is correctly shown when the button of the Activity that triggers Action_ShowDialog_Buy is clicked. But after that, the Dialog itself has a button:

<!-- dialog_buy.xml -->
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent" >

<!-- Other stuff -->

<Button
    android:id="@+id/Button_Buy"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_below="@+id/Some_Other_Stuff"
    android:layout_centerHorizontal="true"
    android:text="@string/button_buy"
    android:onClick="Action_ShowDialog_Buy" />

</RelativeLayout>

The button method Action_ShowDialog_Buy is implemented on the Activity:

public void Action_ShowDialog_Buy(View view) {
    BuyDialog.dismiss() ;
}

but when I click on the Button in the Dialog I receive the error:

java.lang.IllegalStateException: Could not find a method BuyActivity.Action_ShowDialog_Buy(View) in the activity class android.view.ContextThemeWrapper for onClick handler on view class android.widget.Button with id 'Button_Buy'

and below:

Caused by: java.lang.NoSuchMethodException:BuyActivity.Action_ShowDialog_Buy

but as you can see above, the method exists on the Activity.

I think I understand this is some kind of scope issue, but I do not manage to understand it. Please note that I have readed Using onClick attribute in layout xml causes a NoSuchMethodException in Android dialogs but I need to understand, not just to copy code.

Many thanks

Thanks to everyone that tried to help.

I've managed to sort this out by creating a class derived from Dialog and using it, using this code:

import android.app.Dialog;
import android.content.Context;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.widget.RelativeLayout;

public class BuyDialogClass extends Dialog
{

//Ensure this Dialog has a Context we can use
Context mContext ;

public BuyDialogClass(Context context) {
    super(context);
    mContext=context; //Store the Context as provided from caller
}

@Override
 protected void onCreate(final Bundle savedInstanceState)
 {
  super.onCreate(savedInstanceState);
  RelativeLayout ll=(RelativeLayout) LayoutInflater.from(mContext).inflate(R.layout.dialog_buy, null);
  setContentView(ll); 
 }

}

This allowed me to call the dialog as this:

public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_shop);

    initialize_PR();
    display_PR();
    BuyDialog=new BuyDialogClass(this);
    //The setContentView is not necessary here as we call it on the onCreate

    //We can NOT access Dialog widgets from here,
    //because the dialog has not yet been shown.

}
public void Action_ShowDialog_Buy(View view) {
    BuyDialog.show() ;

    //NOW, after showing the dialog, we can access its widgets
    jobject_SeekBar_buy= (SeekBar) BuyDialog.findViewById(R.id.SeekBar_Dialog_Buy) ;
    jobject_SeekBar_buy.setMax(PR_num_coins/currentPR_buy_price) ;
    jobject_SeekBar_buy.setOnSeekBarChangeListener(this);

}
public void Action_Buy_PR(View view) {
    BuyDialog.dismiss() ;
}

I managed to do this by reading Using onClick attribute in layout xml causes a NoSuchMethodException in Android dialogs but I still do not understand this Context issue.