Region Manager can not find region inside of the custom popupwindow
Solution 1:
Probably you need to set the region manager manually, in the popup view's code behind (constructor), like this:
RegionManager.SetRegionName( theNameOfTheContentControlInsideThePopup, WellKnownRegionNames.DataFeedRegion );
RegionManager.SetRegionManager( theNameOfTheContentControlInsideThePopup, theRegionManagerInstanceFromUnity );
You'll have to assign a name to the content control hosting the region and somehow acquire the region manager (ServiceLocator.Current.GetInstance<IRegionManager>()).