Fix NRE when SelectionChanged is firing on a null object. (#717)

* Fix NRE when SelectionChanged is firing on a disposed object.

* Don't forget to hook the SelectionChange up again, after removing everything.

* During control re-use, SelectedValue might be null. So check for that.

1 files changed, 14 insertions, 5 deletions

diff --git a/Xamarin.PropertyEditing.Mac/Controls/PredefinedValuesEditor.cs b/Xamarin.PropertyEditing.Mac/Controls/PredefinedValuesEditor.cs
index 2c80314..f402bf7 100644
--- a/Xamarin.PropertyEditing.Mac/Controls/PredefinedValuesEditor.cs
+++ b/Xamarin.PropertyEditing.Mac/Controls/PredefinedValuesEditor.cs
@@ -44,10 +44,11 @@ namespace Xamarin.PropertyEditing.Mac
 			} else {
 				RequireComboBox ();
 
-				this.comboBox.RemoveAll ();
+				UnhookSelectionChangeAndClearItems ();
 				foreach (var item in ViewModel.PossibleValues) {
 					this.comboBox.Add (new NSString (item));
 				}
+				this.comboBox.SelectionChanged += ComboBox_SelectionChanged;
 			}
 
 			base.OnViewModelChanged (oldModel);
@@ -134,9 +135,7 @@ namespace Xamarin.PropertyEditing.Mac
 		{
 			if (this.comboBox == null)
 				return;
-
-			this.comboBox.SelectionChanged -= ComboBox_SelectionChanged;
-			this.comboBox.RemoveAll ();
+			UnhookSelectionChangeAndClearItems ();
 			this.comboBox.RemoveFromSuperview ();
 			this.comboBox.Dispose ();
 			this.comboBox = null;
@@ -185,9 +184,19 @@ namespace Xamarin.PropertyEditing.Mac
 
 		private void ComboBox_SelectionChanged (object sender, EventArgs e)
 		{
-			if (ViewModel != null && this.comboBox != null) {
+			if (ViewModel != null
+				&& this.comboBox != null
+				&& this.comboBox.SelectedValue != null) {
 				ViewModel.ValueName = this.comboBox.SelectedValue.ToString ();
 			}
 		}
+
+		private void UnhookSelectionChangeAndClearItems ()
+		{
+			if (this.comboBox != null) {
+				this.comboBox.SelectionChanged -= ComboBox_SelectionChanged;
+				this.comboBox.RemoveAll ();
+			}
+		}
 	}
 }