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/face_recognition.lua

@@ -55,6 +55,8 @@ local OUTPUT = dt.configuration.tmp_dir .. PS .. "facerecognition.txt"
 -- namespace
 
 local fc = {}
+fc.module_installed = false
+fc.event_registered = false
 
 -- ensure we meet the minimum api
 du.check_min_api_version("5.0.0", "face_recognition")
@@ -337,6 +339,22 @@ local function face_recognition ()
   end
 end
 
+local function install_module()
+  if not fc.module_installed then
+    dt.register_lib(
+      "face_recognition",     -- Module name
+      _("face recognition"),     -- Visible name
+      true,                -- expandable
+      true,               -- resetable
+      {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 300}},   -- containers
+      fc.widget,
+      nil,-- view_enter
+      nil -- view_leave
+    )
+    fc.module_installed = true
+  end
+end
+
 -- build the interface
 
 fc.unknown_tag = dt.new_widget("entry"){
@@ -466,16 +484,21 @@ fc.widget = dt.new_widget("box"){
   table.unpack(widgets),
 }
 
-dt.register_lib(
-  "face_recognition",     -- Module name
-  _("face recognition"),     -- Visible name
-  true,                -- expandable
-  true,               -- resetable
-  {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 300}},   -- containers
-  fc.widget,
-  nil,-- view_enter
-  nil -- view_leave
-)
+if dt.gui.current_view().name == "lighttable" then
+  install_module()
+else
+  if not fc.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
+    )
+    fc.event_registered = true
+  end
+end
 
 fc.tolerance.value = dt.preferences.read(MODULE, "tolerance", "float")