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/script/script_gui.cpp

@@ -649,6 +649,7 @@ struct ScriptTextfileWindow : public TextfileWindow {
 
 	ScriptTextfileWindow(TextfileType file_type, CompanyID slot) : TextfileWindow(file_type), slot(slot)
 	{
+		this->ConstructWindow();
 		this->OnInvalidateData();
 	}