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/settings_gui.cpp

@@ -97,6 +97,8 @@ struct BaseSetTextfileWindow : public TextfileWindow {
 
 	BaseSetTextfileWindow(TextfileType file_type, const TBaseSet *baseset, StringID content_type) : TextfileWindow(file_type), baseset(baseset), content_type(content_type)
 	{
+		this->ConstructWindow();
+
 		auto textfile = this->baseset->GetTextfile(file_type);
 		this->LoadTextfile(textfile.value(), BASESET_DIR);
 	}