BUG FIX: Set the last document content's DockPanel to null throws exception, due to GiveUpFocus tries to re-display the content again.
BUG FIX: Set the last document content's DockPanel to null throws exception, due to GiveUpFocus tries to re-display the content again.

transistor1/dockpanelsuite,xo-energy/dockpanelsuite,jorik041/dockpanelsuite,15070217668/dockpanelsuite,ArsenShnurkov/dockpanelsuite,rohitlodha/dockpanelsuite,dockpanelsuite/dockpanelsuite,joelbyren/dockpanelsuite,elnomade/dockpanelsuite,compborg/dockpanelsuite,shintadono/dockpanelsuite,thijse/dockpanelsuite,angelapper/dockpanelsuite,RadarNyan/dockpanelsuite,Romout/dockpanelsuite,modulexcite/O2_Fork_dockpanelsuite,15070217668/dockpanelsuite