I upgraded my project from 4.15.2 to 4.17.2 and started getting the same type of errors, but only in my built game.
Error:
Warning: Serialized Class /Script/CoreUObject.LinkerPlaceholderExportObject for a property of WidgetBlueprintGeneratedClass /Game/CCGToolkit/Blueprints/Widgets/Gameplay/ViewCard_Default.ViewCard_Default_C. Reference will be nullptred. Property = ObjectProperty /Game/CCGToolkit/Blueprints/Widgets/Menu/DeckBuilder/LibraryCard.LibraryCard_C:CardBase Item = LinkerPlaceholderExportObject /Game/CCGToolkit/Blueprints/Widgets/Menu/DeckBuilder/DeckBuilder.PLACEHOLDER-INST_of_ViewCard_Default_C_25
Most of my child actors are corrupted in the built game. No errors and problems in Editor.
I am able to restore some functionality via “Create Widget” instead of having static Widgets in my UMG editor.