Load classes before creating label dialog

The "editLabel()" may not pop predefined classes without preloaded "labelHist".

labelImg.py

@@ -106,6 +106,9 @@ class MainWindow(QMainWindow, WindowMixin):
         self.screencastViewer = "firefox"
         self.screencast = "https://youtu.be/p0nR2YsCY_U"
 
+        # Load predefined classes to the list
+        self.loadPredefinedClasses(defaultPrefdefClassFile)
+
         # Main widgets and related state.
         self.labelDialog = LabelDialog(parent=self, listItem=self.labelHist)
 
@@ -396,9 +399,6 @@ class MainWindow(QMainWindow, WindowMixin):
         # Add Chris
         self.difficult = False
 
-        # Load predefined classes to the list
-        self.loadPredefinedClasses(defaultPrefdefClassFile)
-
         self.settings = Settings()
         self.settings.load()
         settings = self.settings