Region Manager can not find region inside of the custom popupwindow

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>()).