title: Android app shutting if form blank - Need a validation form
body: <p>I have a app for convert km/h for mph.
  If the form is blank and press button convert, the app simply closes.
  I understand that i need a function thats validade if the form is blank and show a message informating users that form cannot be blank but i cannot found a code for my problem.</p>
  
  <p>My onClick code is:</p>
  
  <pre><code>        buttonConverter.setOnClickListener(new OnClickListener() {
  
          @Override
          public void onClick(View arg0) {
              double Value1 = Double.valueOf(txtKmhUser.getText().toString());
              double Value2 = Value1 / 1.60;
              Result.setText(String.valueOf(Value2));
  
          }
  
  
      });
  </code></pre>