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/exportLUT.lua

@@ -41,6 +41,11 @@ end
 
 du.check_min_api_version("5.0.0", "exportLUT") 
 
+local eL = {}
+eL.module_installed = false
+eL.event_registered = false
+eL.widgets = {}
+
 -- Thanks Kevin Ertel for this bit
 local os_path_seperator = '/'
 if dt.configuration.running_os == 'windows' then os_path_seperator = '\\' end
@@ -119,27 +124,51 @@ local function export_luts()
   end
 end
 
+local function install_module()
+  if not eL.module_installed then
+    dt.register_lib(
+      _("export haldclut"),
+      _("export haldclut"),
+      true,
+      false,
+      {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 100}},
+      dt.new_widget("box")
+      {
+        orientation = "vertical",
+        table.unpack(eL.widgets),
+      },
+      nil,
+      nil
+    )
+    eL.module_installed = true
+  end
+end
+
 local export_button = dt.new_widget("button"){
   label = _("export"),
   clicked_callback = export_luts
 }
 
-dt.register_lib(
-  _("export haldclut"),
-  _("export haldclut"),
-  true,
-  false,
-  {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 100}},
-  dt.new_widget("box")
-  {
-    orientation = "vertical",
-    identity_label,
-    file_chooser_button,
-    output_label,
-    export_chooser_button,
-    warning_label,
-    export_button
-  },
-  nil,
-  nil
-)
+table.insert(eL.widgets, identity_label)
+table.insert(eL.widgets, file_chooser_button)
+table.insert(eL.widgets, output_label)
+table.insert(eL.widgets, export_chooser_button)
+table.insert(eL.widgets, warning_label)
+table.insert(eL.widgets, export_button)
+
+if dt.gui.current_view().name == "lighttable" then
+  install_module()
+else
+  if not eL.event_registered then
+    dt.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
+    )
+    eL.event_registered = true
+  end
+end
+