XML : put listView child in a hashmap

I'm trying to fetch values from listView with a for loop and put them in a hashmap. but the logcat keeps throwing this error. I can't figure out why. Any help please! the list view has @android:id/list and I have extended ListActivity on my activity here is my code to fetch those values

  listStudent=getListView();  studentMarksList=new ArrayList<HashMap<String,String>>();  String student,obtained_value,max_value;  for (int i=0;i<listStudent.getCount();i++) {      View view=listStudent.getChildAt(i);      studentIdTxt=(TextView) view.findViewById(R.id.student_id_ls);      obtainedTxt=(EditText) view.findViewById(R.id.obtained);      maxTxt=(TextView) view.findViewById(R.id.max);      student=studentIdTxt.getText().toString();      obtained_value=obtainedTxt.getText().toString();      max_value=maxTxt.getText().toString();      //updating the new mark list array      HashMap<String,String>studentMark=new HashMap<String,String>();      studentMark.put(TAG_STUDENT_ID,student);      studentMark.put(TAG_MARKS_OBTAINED,obtained_value);      studentMark.put(TAG_MARKS_MAX,max_value);      studentMarksList.add(studentMark);  }    

And here is my xml layout for the list

  <ListView      android:layout_width="wrap_content"      android:layout_height="wrap_content"      android:id="@android:id/list"      android:layout_alignParentTop="true"      android:layout_alignParentLeft="true"      android:layout_alignParentStart="true"      android:layout_marginTop="106dp" />    

and here is the xml for list items

  <?xml version="1.0" encoding="utf-8"?>  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  android:orientation="vertical" android:layout_width="match_parent"  android:layout_height="match_parent">     <TextView      android:id="@+id/student_name_ls"      android:layout_width="fill_parent"      android:layout_height="wrap_content"      android:paddingBottom="2dip"      android:paddingTop="6dip"      android:textColor="#1e5401"      android:textSize="16sp"      android:textStyle="bold" />    <!-- Marks label -->   <EditText      android:id="@+id/obtained"      android:layout_width="fill_parent"      android:layout_height="wrap_content"      android:paddingBottom="2dip"      android:textColor="#acacac"      android:inputType="numberDecimal"      android:numeric="decimal" />   <TextView      android:id="@+id/max"      android:layout_width="fill_parent"      android:layout_height="wrap_content"      android:paddingBottom="2dip"      android:textColor="#0b0b0b"      android:enabled="false"      android:editable="false" />   <TextView      android:id="@+id/student_id_ls"      android:layout_width="fill_parent"      android:layout_height="wrap_content"      android:paddingBottom="2dip"      android:textColor="#060606"      android:visibility="gone" />     </LinearLayout>    

And the problem is on this line "studentIdTxt=(TextView) view.findViewById(R.id.student_id_ls);" throws nullpointerexception any help please