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/HDRMerge.lua

@@ -94,6 +94,10 @@ local GUI = { --GUI Elements Table
     }
 }
 
+HDRM.module_installed = false
+HDRM.event_registered = false
+
+
 --Detect User Styles--
 local styles = dt.styles
 local styles_count = 1 -- 'none' = 1
@@ -264,6 +268,23 @@ local function main()
 
 end
 
+local function install_module()
+  if not HDRM.module_installed then
+    dt.register_lib( -- register HDRMerge module
+        'HDRMerge_Lib', -- Module name
+        _('HDRMerge'), -- name
+        true,   -- expandable
+        true,   -- resetable
+        {[dt.gui.views.lighttable] = {'DT_UI_CONTAINER_PANEL_RIGHT_CENTER', 99}},   -- containers
+        dt.new_widget('box'){
+            orientation = 'vertical',
+            GUI.stack
+        }
+    )
+    HDRM.module_installed = true
+  end
+end
+
 -- GUI Elements --
 local lbl_hdr = dt.new_widget('section_label'){
     label = _('HDRMerge options')
@@ -411,14 +432,18 @@ else
     GUI.stack.active = 2
 end
 
-dt.register_lib( -- register HDRMerge module
-    'HDRMerge_Lib', -- Module name
-    _('HDRMerge'), -- name
-    true,   -- expandable
-    true,   -- resetable
-    {[dt.gui.views.lighttable] = {'DT_UI_CONTAINER_PANEL_RIGHT_CENTER', 99}},   -- containers
-    dt.new_widget('box'){
-        orientation = 'vertical',
-        GUI.stack
-        }
-)
+if dt.gui.current_view().name == "lighttable" then
+  install_module()
+else
+  if not HDRM.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
+    )
+    HDRM.event_registered = true
+  end
+end