Fix: TextfileWindow called virtual methods before constructor completed. (#11889)

SetStringParameters() was called during widget tree init in the constructor.

Calls within a constructor cannot call the derived classes methods. This would result in invalid data being passed to the string system, which could then crash.

(cherry picked from commit fef0bfcfd3)

src/help_gui.cpp

@@ -59,6 +59,8 @@ static std::optional<std::string> FindGameManualFilePath(std::string_view filena
 struct GameManualTextfileWindow : public TextfileWindow {
 	GameManualTextfileWindow(std::string_view filename) : TextfileWindow(TFT_GAME_MANUAL)
 	{
+		this->ConstructWindow();
+
 		/* Mark the content of these files as trusted. */
 		this->trusted = true;