change point to default, because a point cannot be null

Author: Hirogen

src/LogExpert/Controls/LogWindow/LogWindowsPublic.cs

@@ -873,7 +873,7 @@ public void ToggleBookmark()
             if (filterGridView.Focused)
             {
                 gridView = filterGridView;
-                if (gridView.CurrentCellAddress == null || gridView.CurrentCellAddress.Y == -1)
+                if (gridView.CurrentCellAddress == default || gridView.CurrentCellAddress.Y == -1)
                 {
                     return;
                 }
@@ -883,7 +883,7 @@ public void ToggleBookmark()
             else
             {
                 gridView = dataGridView;
-                if (gridView.CurrentCellAddress == null || gridView.CurrentCellAddress.Y == -1)
+                if (gridView.CurrentCellAddress == default || gridView.CurrentCellAddress.Y == -1)
                 {
                     return;
                 }