Add environment object to previews so that they work.

Multiplatform/Shared/SceneNavigationView.swift

@@ -35,5 +35,6 @@ struct SceneNavigationView: View {
 struct NavigationView_Previews: PreviewProvider {
     static var previews: some View {
 		SceneNavigationView()
+			.environmentObject(SceneModel())
     }
 }

Multiplatform/Shared/Sidebar/SidebarView.swift

@@ -37,5 +37,6 @@ struct SidebarView: View {
 struct SidebarView_Previews: PreviewProvider {
     static var previews: some View {
         SidebarView()
+			.environmentObject(SceneModel())
     }
 }