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/AutoGrouper.lua

@@ -45,6 +45,17 @@ local function _(msgid)
     return gettext.dgettext("AutoGrouper", msgid)
 end
 
+local Ag = {}
+Ag.module_installed = false
+Ag.event_registered = false
+
+local GUI = {
+    gap =           {},
+    selected =      {},
+    collection =    {}
+}
+
+
 local function InRange(test, low, high) --tests if test value is within range of low and high (inclusive)
     if test >= low and test <= high then
         return true
@@ -111,12 +122,26 @@ local function main(on_collection)
     end
 end
 
+local function install_module()
+  if not Ag.module_installed then
+    dt.register_lib(
+        'AutoGroup_Lib',    -- Module name
+        _('auto group'),    -- name
+        true,   -- expandable
+        true,   -- resetable
+        {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 99}},   -- containers
+        dt.new_widget("box"){
+            orientation = "vertical",
+            GUI.gap,
+            GUI.selected,
+            GUI.collection
+            }
+    )
+    Ag.module_installed = true
+  end
+end
+
 -- GUI --
-GUI = {
-    gap =           {},
-    selected =      {},
-    collection =    {}
-}
 temp = dt.preferences.read(MOD, 'active_gap', 'integer')
 if not InRange(temp, 1, 86400) then temp = 3 end
 GUI.gap = dt.new_widget('slider'){
@@ -143,16 +168,20 @@ GUI.collection = dt.new_widget("button"){
     tooltip =_('auto group the entire collection'),
     clicked_callback = function() main(true) end
 }
-dt.register_lib(
-    'AutoGroup_Lib',    -- Module name
-    _('auto group'),    -- name
-    true,   -- expandable
-    true,   -- resetable
-    {[dt.gui.views.lighttable] = {"DT_UI_CONTAINER_PANEL_RIGHT_CENTER", 99}},   -- containers
-    dt.new_widget("box"){
-        orientation = "vertical",
-        GUI.gap,
-        GUI.selected,
-        GUI.collection
-        }
-)
+
+if dt.gui.current_view().name == "lighttable" then
+  install_module()
+else
+  if not Ag.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
+    )
+    Ag.event_registered = true
+  end
+end
+