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/network/network_content_gui.cpp

@@ -44,6 +44,8 @@ struct ContentTextfileWindow : public TextfileWindow {
 
 	ContentTextfileWindow(TextfileType file_type, const ContentInfo *ci) : TextfileWindow(file_type), ci(ci)
 	{
+		this->ConstructWindow();
+
 		const char *textfile = this->ci->GetTextfile(file_type);
 		this->LoadTextfile(textfile, GetContentInfoSubDir(this->ci->type));
 	}