fix an crash in SwiftUI preview

SessionUIKit/Utilities/SwiftUI+Utilities.swift

@@ -10,7 +10,9 @@ struct ViewControllerHolder {
 
 struct ViewControllerKey: EnvironmentKey {
     static var defaultValue: ViewControllerHolder {
-        return ViewControllerHolder(value: CurrentAppContext().mainWindow?.rootViewController)
+        return ViewControllerHolder(
+            value: HasAppContext() ? CurrentAppContext().mainWindow?.rootViewController : nil
+        )
     }
 }