Added check to ensure we are in lighttable view prior to callling darktable.register_lib. If darktable was invoked in single image (darkroom) mode, then an event handler is registered to detect the change from darkroom mode to lighttable mode so that we can then call darktable.register_lib.

contrib/transfer_hierarchy.lua

@@ -99,6 +99,10 @@ end
 
 -- Helper functions: BEGIN
 
+local th = {}
+th.module_installed = false
+th.event_registered = false
+
 local function pathExists(path)
    local success, err, errno = os.rename(path, path)
    if not success then
@@ -122,6 +126,16 @@ local function createDirectory(path)
    end
 end
 
+local function install_module()
+  if not th.module_installed then
+    darktable.register_lib(LIB_ID,
+               "transfer hierarchy", true, true, {
+            [darktable.gui.views.lighttable] = { "DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 700 }
+               }, th.transfer_widget, nil, nil)
+    th.module_installed = true
+  end
+end
+
 -- Helper functions: END
 
 
@@ -292,7 +306,7 @@ end
 
 
 
-local transfer_widget = darktable.new_widget("box") {
+th.transfer_widget = darktable.new_widget("box") {
    orientation = "vertical",
    darktable.new_widget("button") {
      label = _("calculate"),
@@ -347,12 +361,18 @@ darktable.preferences.register(
 
 -- Preferences: END
 
-
-
-
-
-
-darktable.register_lib(LIB_ID,
-		       "transfer hierarchy", true, true, {
-			  [darktable.gui.views.lighttable] = { "DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 700 }
-		       }, transfer_widget, nil, nil)
+if darktable.gui.current_view().name == "lighttable" then
+  install_module()
+else
+  if not th.event_registered then
+    darktable.register_event(
+      "view-changed",
+      function(event, old_view, new_view)
+        if new_view.name == "lighttable" and old_view.name == "darkroom" then
+          install_module()
+         end
+      end
+    )
+    th.event_registered = true
+  end
+end