Fix dismissing a sheet (issue #6)

When dismissing a sheet, the sheets ContentView sometimes
already reflects that change in presentation while the
View slides off screen. Showing an error.

Now we keep a handle to the actual VC being presented.

Sources/ViewController/Presentations/AutoPresentation.swift

@@ -26,19 +26,48 @@ internal struct AutoPresentationViewModifier<VC>: ViewModifier
     @ObservedObject var presentingViewController : VC
     let mode : ViewControllerPresentationMode
 
-    var body: some View {
+    // Keep a handle to the VC being presented. We do this to avoid issue #6,
+    // i.e. when a sheet is dismissed and transitions off-screen, the
+    // "presentedViewController" is already gone (dismissed).
+    // The `body` of this `Present` View would then evaluate to the
+    // `TypeMismatchInfoView` during the dismiss.
+    // So we keep the VC being presented around, to make sure we still have a
+    // handle for the content-view while it is being dismissed.
+    @State private var viewController : _ViewController?
+
+    private var activeVC: _ViewController? {
+      if let activeVC = viewController { return activeVC }
+
       if let presentation =
-          presentingViewController.activePresentation(for: mode)
+               presentingViewController.activePresentation(for: mode)
       {
-        let presentedViewController = presentation.viewController
+        // Note: Do not modify `@State` in here! (i.e. do not push to the
+        //       `viewController` variable as part of the evaluation)
+        // This happens if the VC is getting presented.
+        return presentation.viewController
+      }
+
+      return nil
+    }
+
+    var body: some View {
+      if let presentedViewController = activeVC {
         presentedViewController.anyControlledContentView
           .environment(\.viewControllerPresentationMode, mode)
           .navigationTitle(presentedViewController.navigationTitle)
+          .onAppear {
+            viewController = presentedViewController
+          }
+          .onDisappear { // This seems to be a proper onDidDisappear
+            viewController = nil
+          }
       }
       else {
-        TypeMismatchInfoView<AnyViewController, VC>(
-          parent: presentingViewController, expectedMode: mode
-        )
+        #if DEBUG
+          TypeMismatchInfoView<AnyViewController, VC>(
+            parent: presentingViewController, expectedMode: mode
+          )
+        #endif
       }
     }
   }