Fixed an issue when importing tags

Fixed an issue where if the link generation returned None a crash would occur.

ArtNet/gui/importer_window.py

@@ -371,14 +371,14 @@ class ImporterWindow(ArtnetMainWindow):
         """
         self.ui.link_label.setText(ImporterWindow.UNKNOWN_LINK)
 
-        if validators.url(link):
+        if link is not None and validators.url(link):
             self.curr_link = link
             self.data_changed = True
             hyperlink = "<a href=\"{0}\">{1}</a>".format(link, "Source")
             self.ui.link_label.setText(hyperlink)
             self.ui.link_label.setToolTip(link)
             return link
-        elif len(link) == 0:
+        elif link is None or len(link) == 0:
             return ""
         else:
             self.ui.link_label.setText(ImporterWindow.UNKNOWN_LINK)