Release event listeners on close

- Issue MatterHackers/MCCentral#3016
Switching colors leaks GuiWidgets

ApplicationView/PrinterModels.cs

@@ -764,8 +764,6 @@ namespace MatterHackers.MatterControl
 			this.Connection.ReadLineReplacementString = this.Settings.GetValue(SettingsKey.read_regex);
 		}
 
-		
-
 		public PrinterViewState ViewState { get; } = new PrinterViewState();
 
 		private PrinterSettings _settings;

PartPreviewWindow/PrinterTabPage.cs

@@ -97,10 +97,7 @@ namespace MatterHackers.MatterControl.PartPreviewWindow
 				}
 			};
 
-			printer.ViewState.ViewModeChanged += (s, e) =>
-			{
-				this.SetViewMode(e.ViewMode);
-			};
+			printer.ViewState.ViewModeChanged += ViewState_ViewModeChanged;
 
 			layerScrollbar = new SliceLayerSelector(printer, sceneContext)
 			{
@@ -209,6 +206,11 @@ namespace MatterHackers.MatterControl.PartPreviewWindow
 
 		}
 
+		private void ViewState_ViewModeChanged(object sender, ViewModeChangedEventArgs e)
+		{
+			this.SetViewMode(e.ViewMode);
+		}
+
 		private void ConfigurePrinter_Changed(object sender, EventArgs e)
 		{
 			this.ProcessOptionalTab();
@@ -400,6 +402,7 @@ namespace MatterHackers.MatterControl.PartPreviewWindow
 
 			sceneContext.LoadedGCodeChanged -= BedPlate_LoadedGCodeChanged;
 			printer.ViewState.ConfigurePrinterChanged -= ConfigurePrinter_Changed;
+			printer.ViewState.ViewModeChanged -= ViewState_ViewModeChanged;
 
 			base.OnClosed(e);
 		}

PartPreviewWindow/SliceLayerSelector.cs

@@ -200,7 +200,6 @@ namespace MatterHackers.MatterControl.PartPreviewWindow
 				set => layerSlider.Value = value;
 			}
 
-
 			public override void OnBoundsChanged(EventArgs e)
 			{
 				base.OnBoundsChanged(e);
@@ -212,6 +211,13 @@ namespace MatterHackers.MatterControl.PartPreviewWindow
 				}
 			}
 
+			public override void OnClosed(ClosedEventArgs e)
+			{
+				base.OnClosed(e);
+
+				sceneContext.ActiveLayerChanged -= ActiveLayer_Changed;
+			}
+
 			private void ActiveLayer_Changed(object sender, EventArgs e)
 			{
 				if (layerSlider != null