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A NullReferenceException is occasionally logged at roughly line 526 in DockContentHandler:
if (Pane != null && DockState == Pane.DockState)
if ((Pane != oldPane) ||
(Pane == oldPane && oldDockState != oldPane.DockState)) << logs indicate exception on this line
// Avoid early refresh of hidden AutoHide panes
if ((Pane.DockWindow == null || Pane.DockWindow.Visible || Pane.IsHidden) && !Pane.IsAutoHide)
It seems this would only happen if oldPane was null, but logically we cannot get into that state (Pane cannot be null, so if oldPane is null, the DockState will not be checked).
Closing as not an issue. The problem was that the DockContent was being hidden after it had been disposed. This is not a valid use case.