title: Checking if node exists and adding child node
body: <p>Since it's impossible to make nodes invisible, I decided to don't create them until I'll need them. </p>
  
  <p>My code:</p>
  
  <pre><code>if(comboBox3.Text == "Books")
  {
     if (treeView1.Nodes.ContainsKey("Books") == true)
     {
        treeView1.Nodes["Books"].Nodes.Add(textBox1.Text);
     }
  
     else if (treeView1.Nodes.ContainsKey("Books") == false)
     {
        treeView1.Nodes.Add("Books");
        treeView1.Nodes["Books"].Nodes.Add(textBox1.Text);
     }
  
  }
  </code></pre>
  
  <p>In <code>ComboBox</code> there's few categories. It's code that is responsible for "Books". In TextBox I'm writing title and after clicking a button, this code is starting to work. First, it is checking if root node "Books" exists. If it is, it just add anything that is inside textbox as child node. But if not, it creates root node called "Books" and after that adds a child node. Im getting error in this line:</p>
  
  <pre><code>treeView1.Nodes["Books"].Nodes.Add(textBox1.Text);
  </code></pre>
  
  <p>Error:</p>
  
  <pre><code>NullReferenceException was unhandled
  </code></pre>
  
  <p>Also, is that possible to change index number, ie. I want to make 5 categories, but also I want to have specific order, let's say 1.House 2.Cars 3.Books 4.Phones 5.Bikes and I want to add first Books, then Cars, an then Bikes. Is there anything to change index number?</p>