Fixed the bug where resising the window and undo or clear crashes

Paint_2/DotPencil.cs

@@ -12,7 +12,7 @@ using System.Drawing;
 
 namespace Paint_2
 {
-    public class DotPencil:PaintTool
+    public class DotPencil : PaintTool
     {
         private List<List<Point>> _drawings;
         private List<List<Point>> _drawingsRedo;

Paint_2/Form1.cs

@@ -89,7 +89,7 @@ namespace Paint_2
         private void ForceRefresh()
         {
             canvas.Image = sketch.ForcePaint();
-            RefreshUi();
+            //RefreshUi();
         }
         private void RefreshUi()
         {

Paint_2/Pencil.cs

@@ -12,7 +12,7 @@ using System.Threading.Tasks;
 
 namespace Paint_2
 {
-    internal class Pencil : PaintTool
+    public class Pencil : PaintTool
     {
         private List<List<Point>> _drawings;
         private List<List<Point>> _drawingsRedo;

Paint_2/Sketch.cs

@@ -52,6 +52,7 @@ namespace Paint_2
             if (newSize.Width > 0 && newSize.Height > 0)
             {
                 Drawing = new Bitmap(newSize.Width, newSize.Height);
+                SketchSize = newSize;
             }
         }
         public void ChangeTool(int toolId)