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/LabelsToTags.lua

@@ -55,6 +55,10 @@ du.check_min_api_version("3.0.0", "LabelsToTags")
 -- Lua 5.3 no longer has "unpack" but "table.unpack"
 unpack = unpack or table.unpack
 
+local ltt = {}
+ltt.module_installed = false
+ltt.event_registered = false
+
 local LIB_ID = "LabelsToTags"
 
 -- Helper functions: BEGIN
@@ -184,7 +188,7 @@ local function doTagging(selfC)
    job.valid = false
 end
 
-local my_widget = darktable.new_widget("box") {
+ltt.my_widget = darktable.new_widget("box") {
    orientation = "vertical",
    mappingComboBox,
    darktable.new_widget("button") {
@@ -217,6 +221,15 @@ darktable.register_tag_mapping = function(name, mapping)
    mappingComboBox.reset_callback(mappingComboBox)
 end
 
+local function install_module()
+  if not ltt.module_installed then
+   darktable.register_lib(LIB_ID,"labels to tags",true,true,{
+              [darktable.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER",20},
+                            },ltt.my_widget,nil,nil)
+    ltt.module_installed = true
+  end
+end
+
 --[[
 darktable.register_tag_mapping("Example",
 			       { ["+----*"] = { "Red", "Only red" },
@@ -229,6 +242,19 @@ darktable.register_tag_mapping("Example",
 				 ["*****R"] = { "Rejected" } })
 ]]
 
-darktable.register_lib(LIB_ID,"labels to tags",true,true,{
-			  [darktable.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER",20},
-								 },my_widget,nil,nil)
+if darktable.gui.current_view().name == "lighttable" then
+  install_module()
+else
+  if not ltt.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
+    )
+    ltt.event_registered = true
+  end
+end
+