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This specific case failed when the topmost window in the window stack (ie. the first to be attempted to be redrawn on reentering the desktop) had the focus and thus attempts to enable Ursula tinting.
Though a check was made to see if the first sprite (from which the tint is derived) was present, no check was made to see if the tool_list was present => dereferenced NULL pointer.
Retagged as Wimp-5_29.