trying to fix undo bug

io/previewgenerator.cpp

@@ -1,6 +1,6 @@
 #include "previewgenerator.h"
 
-PreviewGenerator::PreviewGenerator(QWidget* parent) : QThread(parent)
+PreviewGenerator::PreviewGenerator(QObject* parent) : QThread(parent)
 {
 
 }

io/previewgenerator.h

@@ -6,7 +6,7 @@
 class PreviewGenerator : public QThread
 {
 public:
-    PreviewGenerator(QWidget* parent = 0);
+    PreviewGenerator(QObject* parent = 0);
     void run() override;
 };