Handle empty text when double-clicking TextBox.

Fixes #458

src/Perspex.Controls/TextBox.cs

@@ -373,23 +373,26 @@ namespace Perspex.Controls
                 var index = CaretIndex = _presenter.GetCaretIndex(point);
                 var text = Text;
 
-                switch (e.ClickCount)
+                if (text != null)
                 {
-                    case 1:
-                        SelectionStart = SelectionEnd = index;
-                        break;
-                    case 2:
-                        if (!StringUtils.IsStartOfWord(text, index))
-                        {
-                            SelectionStart = StringUtils.PreviousWord(text, index, false);
-                        }
-
-                        SelectionEnd = StringUtils.NextWord(text, index, false);
-                        break;
-                    case 3:
-                        SelectionStart = 0;
-                        SelectionEnd = text.Length;
-                        break;
+                    switch (e.ClickCount)
+                    {
+                        case 1:
+                            SelectionStart = SelectionEnd = index;
+                            break;
+                        case 2:
+                            if (!StringUtils.IsStartOfWord(text, index))
+                            {
+                                SelectionStart = StringUtils.PreviousWord(text, index, false);
+                            }
+
+                            SelectionEnd = StringUtils.NextWord(text, index, false);
+                            break;
+                        case 3:
+                            SelectionStart = 0;
+                            SelectionEnd = text.Length;
+                            break;
+                    }
                 }
 
                 e.Device.Capture(_presenter);

src/Perspex.Controls/Utils/StringUtils.cs

@@ -21,6 +21,11 @@ namespace Perspex.Controls.Utils
 
         public static bool IsStartOfWord(string text, int index)
         {
+            if (index >= text.Length)
+            {
+                return false;
+            }
+
             // A 'word' starts with an AlphaNumeric or some punctuation symbols immediately
             // preceeded by lwsp.
             if (index > 0 && !char.IsWhiteSpace(text[index - 1]))