@userMod2 Check below solutions -
When you use 
myList.getAdapter().getView(i,null,null)
you are getting a new instance of the item view, try the 
ListView.getChildAt(position)
method like this
 private void ButtonClick() {
    /** get all values of the EditText-Fields */
    View v;
    ArrayList<String> mannschaftsnamen = new ArrayList<String>();
    EditText et;
    for (int i = 0; i < myList.getCount(); i++) {
      v = myList.getAdapter().getView(i, null, null);
      et = (EditText) v.findViewById(i);
      mannschaftsnamen.add(et.getText().toString());
    }
    // Add your action code here
    // Add your action code here
 }
- Solution 2 - public void onScroll(AbsListView v, int firstVisibleItem, int   visibleCount, int totalItemCount)
{
ListView lv = this.getListView();
int childCount = lv.getChildCount();
for (int i = 0; i < childCount; i++)
{
    View v = lv.getChildAt(i);
    TextView tx = (TextView) v.findViewById(R.id.mytext);
    tx.setTextSize(textSize);
}
}