ComboBox cursonDownKey nullref fix (#1472)

* added null guard to fix null ref when pressing keyDown inside combobox
Improved an error message when view cannot be found

* Added a unit test to ensure combobox can process all key events
Found and fixed a new nullref

* Found a new bug when source is already present and combobox is added to a top view

* searchSet is auto initialized to new List() now to make the code a little bit safer

UnitTests/ComboBoxTests.cs

@@ -0,0 +1,24 @@
+using System;
+using System.Linq;
+using Terminal.Gui;
+using Xunit;
+
+namespace UnitTests {
+    public class ComboBoxTests {
+        [Fact]
+        [AutoInitShutdown]
+        public void EnsureKeyEventsDoNotCauseExceptions ()
+        {
+            var comboBox = new ComboBox ("0");
+
+            var source = Enumerable.Range (0, 15).Select (x => x.ToString ()).ToArray ();
+            comboBox.SetSource(source);
+
+            Application.Top.Add(comboBox);
+
+            foreach (var key in (Key [])Enum.GetValues (typeof(Key))) {
+                comboBox.ProcessKey (new KeyEvent (key, new KeyModifiers ()));
+            }
+        }
+    }
+}