title: TargetInvocationException occurred, but I have absolutely no idea why
body: <p>I'm getting this unhandled exception error:</p>
  
  <blockquote>
    <p>An unhandled exception of type 'System.Reflection.TargetInvocationException' occurred in PresentationFramework.dll.<br><br>
    Additional information: Exception has been thrown by the target of an invocation.</p>
  </blockquote>
  
  <p>This is my code in short. It's a calculator of two TextBoxes which should together become a new answer when the user press +, -, /, x button made in <a href="http://en.wikipedia.org/wiki/Windows_Presentation_Foundation" rel="nofollow noreferrer">WPF</a>.</p>
  
  <pre><code>public partial class MainWindow : Window
  {
      public string numberInString
      {
          get { return TextDoos.Text; }
          set { TextDoos.Text = value; }
      }
  
      public MainWindow()
      {
          if(TextDoos.Text == "")
          {
              if(TextDoos2.Text == "")
              {
                  RekenFunctie(TextDoos.Text, TextDoos2.Text);
              }
          }
      }
  }
  
  public int RekenFunctie(string numberInString, string numberInString)
  {
      int antwoord;
      int getal = Convert.ToInt32(numberInString);
      int getal2 = Convert.ToInt32(numberInString2);
  
      if (Buttons.IsPressed) // This is the + button, there are also -,x,/ buttons.
      {
         antwoord = getal + getal2;
         return antwoord;
      }
  }
  </code></pre>
  
  <p>I can't see why it isn't working...</p>