Fix PostEvent() segfault (#403)

Fix segfault when PostEvent() called on inactive screen.

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

src/ftxui/component/screen_interactive.cpp

@@ -295,10 +295,14 @@ ScreenInteractive ScreenInteractive::FitComponent() {
 }
 
 void ScreenInteractive::Post(Task task) {
-  if (!quit_) {
-    task_sender_->Send(std::move(task));
-  }
+  // Task/Events sent toward inactive screen or screen waiting to become
+  // inactive are dropped.
+  if (!task_sender_)
+    return;
+
+  task_sender_->Send(std::move(task));
 }
+
 void ScreenInteractive::PostEvent(Event event) {
   Post(event);
 }