title: Nulling an Entry results in crash, TextChanged
body: <p>I have an entry that collects a number that is used afterward as a double.
  It works all fine until one deletes all numbers in the entry to enter a new one.</p>
  
  <p>I tried fixing it by throwing a new Exception as soon as the Text is null but it still crashes with </p>
  
  <blockquote>
    <p>Input string was not in a correct format. (FormatException)</p>
  </blockquote>
  
  <p>the error occurs in <code>moneyOutput.Text = Convert.ToDouble(moneyInput.Text).ToString() + "€";</code></p>
  
  <p>here is my xaml code:</p>
  
  <pre><code>&lt;ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
           xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
           x:Class="Bierrechner.Priceentry"
           BackgroundColor="#8c1029"
           Title="Eingabe"&gt;
  &lt;ScrollView&gt;
      &lt;StackLayout Margin="0,30,0,0" &gt;
  
  
          &lt;Entry x:Name="moneyInput"  TextChanged="MoneyInput_TextChanged" Placeholder="z.B. 34,99" PlaceholderColor="Gray" TextColor="White" Keyboard="Numeric"/&gt;
          &lt;Label x:Name="moneyOutput" Margin="0,40,0,0" HorizontalTextAlignment="Center" FontSize="Large" TextColor="Orange"/&gt;
          &lt;Button Text="Weiter" Clicked="Button_Clicked" HorizontalOptions="Center" VerticalOptions="EndAndExpand"/&gt;
  
  
      &lt;/StackLayout&gt;
  &lt;/ScrollView&gt;
  </code></pre>
  
  <p></p>
  
  <p>and my xaml.cs code:</p>
  
  <pre><code>private void MoneyInput_TextChanged(object sender, TextChangedEventArgs e)
      {
          if (moneyInput.Text != null)
          {
              moneyOutput.Text = Convert.ToDouble(moneyInput.Text).ToString() + "€";
              sharePrice = Convert.ToDouble(moneyInput.Text);
          }
  
          else if (moneyInput.Text == null) 
          {
              throw new ArgumentException("String cannot be null or empty");                
          }
      }
  </code></pre>
  
  <p>I noticed that the <code>if (moneyInput.Text != null)</code> doesn't have any effect because the code is still executed.</p>