Tooltip crash [Watson] clr20r3: NULL_POINTER_READ_c0000005_PresentationFramework.dll!System.Windows.Controls.PopupControlService.OnPostProcessInput

[SamBent]

NRE when closing tooltips in some situations, due to re-entrancy.

[Port of servicing fix for .NET 4.8]

[SamBent]

Fix ported to 3.1.19, 5.0.0. (The offending code is completely rewritten in 6.0.)