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.

src/newgrf_gui.cpp

@@ -560,6 +560,8 @@ struct NewGRFTextfileWindow : public TextfileWindow {
 
 	NewGRFTextfileWindow(TextfileType file_type, const GRFConfig *c) : TextfileWindow(file_type), grf_config(c)
 	{
+		this->ConstructWindow();
+
 		auto textfile = this->grf_config->GetTextfile(file_type);
 		this->LoadTextfile(textfile.value(), NEWGRF_DIR);
 	}