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/ext_editor.lua

@@ -84,6 +84,12 @@ du.check_min_api_version("5.0.2", MODULE_NAME)  -- darktable 3.x
 -- OS compatibility
 local PS = dt.configuration.running_os == "windows" and  "\\"  or  "/"
 
+-- namespace
+local ee = {}
+ee.module_installed = false
+ee.event_registered = false
+ee.widgets = {}
+
 
 -- translation
 local gettext = dt.gettext
@@ -329,6 +335,26 @@ local function export2collection(storage, image_table, extra_data)
   dt.print (_("finished exporting"))
   end
 
+-- install the module in the UI
+local function install_module()
+  if not ee.module_installed then
+    -- register new module "external editors" in lighttable ------------------------
+    dt.register_lib(
+      MODULE_NAME,          
+      _("external editors"),  
+      true, -- expandable
+      false,  -- resetable
+      {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 100}},  
+      dt.new_widget("box") {
+        orientation = "vertical",
+        table.unpack(ee.widgets),
+        },
+      nil,  -- view_enter
+      nil   -- view_leave
+      )
+    ee.module_installed = true
+  end
+end
 
 -- combobox, with variable number of entries ----------------------------------
 local combobox = dt.new_widget("combobox") {
@@ -381,22 +407,26 @@ local box1 = dt.new_widget("box") {
   button_update_list
   }
 
+table.insert(ee.widgets, combobox)
+table.insert(ee.widgets, box1)
 
 -- register new module "external editors" in lighttable ------------------------
-dt.register_lib(
-  MODULE_NAME,          
-  _("external editors"),  
-  true, -- expandable
-  false,  -- resetable
-  {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 100}},  
-  dt.new_widget("box") {
-    orientation = "vertical",
-    combobox,
-    box1
-    },
-  nil,  -- view_enter
-  nil   -- view_leave
-  )
+if dt.gui.current_view().name == "lighttable" then
+  install_module()
+else
+  if not ee.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
+    )
+    ee.event_registered = true
+  end
+end
+
 
 
 -- initialize list of programs and widgets ------------------------------------