Why 'unfortunately my application has stopped ' in android emulator is appearing

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Java

Mobile

Android

My App works and appears on the screen in the emulator. I get 2 numbers for num1 and num2 and use the 'Add' button and show the answer in 'answer'. I am able to enter a number in num1 but unable to enter for num2 as I could not place the cursor on num2.

If I click the 'Add' button it throws an error stating 'unfortunately app stopped working'. In the design mode I have checked the 'clickable' property and in 'onClick' I have also selected the 'onButtonClick'.

public class MainActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

    }
     public void onButtonClick(View v)
     {
         int n1,n2,ans=0;
         EditText e1=(EditText) findViewById(R.id.num1);
         EditText e2=(EditText) findViewById(R.id.num2);
        TextView view_answer=(TextView) findViewById(R.id.answer);
         n1=Integer.parseInt(e1.getText().toString());
         n2=Integer.parseInt(e2.getText().toString());
        ans=n1+n2;
        view_answer.setText(Integer.toString (ans));
     }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }


}

<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" >

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Add "
        android:id="@+id/add"
        android:layout_centerVertical="true"
        android:layout_centerHorizontal="true"
        android:clickable="true"
        android:nestedScrollingEnabled="false"
        android:/>

    <textview>
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"

        android:id="@+id/answer"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="90dp"
        android:width="123dp"
        android:removed="#ffcbffff" />

    <edittext>
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/num1"
        android:layout_alignParentTop="true"
        android:layout_alignRight="@+id/add"
        android:layout_alignEnd="@+id/add"
        android:layout_marginTop="45dp"
        android:width="65dp"
        android:removed="#fffeffa0" />

    <edittext>
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/num2"
        android:width="65dp"
        android:removed="#ffffa6f0"
        android:layout_below="@+id/num1"
        android:layout_centerHorizontal="true"
        android:textSize="20dp"
        android:textIsSelectable="true"
        android:height="45dp"
        android:layout_marginTop="88dp" />
</edittext></edittext></textview></relativelayout>


<resources>

    <string name="app_name">My Application</string>
    <string name="hello_world">Hello world!</string>
    <string name="action_settings">Settings</string>
    <string name="title_activity_main">MainActivity</string>

    <string name="num1"></string>
    <string name="num2"></string>
    <string name="answer"></string>
    <string name="add">Add</string>

</resources>