Fix opening empty string

src/download.cpp

@@ -47,7 +47,7 @@ QString Download::load(QString f) {
 
 	if (reply->error() != QNetworkReply::NoError || !reply->isReadable()) {
 		cerr << reply->errorString().toStdString() << endl;
-		return NULL;
+		return QString();
 	}
 
 	// store data in tempfile

src/main.cpp

@@ -99,7 +99,7 @@ int main(int argc, char *argv[]) {
 		} else {
 			file = QString::fromUtf8(argv[optind]);
 		}
-		if (file == NULL) {
+		if (file.isNull()) {
 			return 1;
 		}
 	}