Fix grayed out preview generation button for light objects

Mistake in e7bea3fb6e.

We should only skip preview generation for objects that don't support
preview rendering, not completely forbid accessing preview data of such
IDs.

source/blender/blenkernel/intern/icons.cc

@@ -359,30 +359,22 @@ void BKE_previewimg_id_copy(ID *new_id, const ID *old_id)
 PreviewImage **BKE_previewimg_id_get_p(const ID *id)
 {
   switch (GS(id->name)) {
-    case ID_OB: {
-      Object *ob = (Object *)id;
-      /* Currently, only object types with real geometry can be rendered as preview. */
-      if (!OB_TYPE_IS_GEOMETRY(ob->type)) {
-        return nullptr;
-      }
-      return &ob->preview;
-    }
-
 #define ID_PRV_CASE(id_code, id_struct) \
   case id_code: { \
     return &((id_struct *)id)->preview; \
   } \
     ((void)0)
-      ID_PRV_CASE(ID_MA, Material);
-      ID_PRV_CASE(ID_TE, Tex);
-      ID_PRV_CASE(ID_WO, World);
-      ID_PRV_CASE(ID_LA, Light);
-      ID_PRV_CASE(ID_IM, Image);
-      ID_PRV_CASE(ID_BR, Brush);
-      ID_PRV_CASE(ID_GR, Collection);
-      ID_PRV_CASE(ID_SCE, Scene);
-      ID_PRV_CASE(ID_SCR, bScreen);
-      ID_PRV_CASE(ID_AC, bAction);
+    ID_PRV_CASE(ID_OB, Object);
+    ID_PRV_CASE(ID_MA, Material);
+    ID_PRV_CASE(ID_TE, Tex);
+    ID_PRV_CASE(ID_WO, World);
+    ID_PRV_CASE(ID_LA, Light);
+    ID_PRV_CASE(ID_IM, Image);
+    ID_PRV_CASE(ID_BR, Brush);
+    ID_PRV_CASE(ID_GR, Collection);
+    ID_PRV_CASE(ID_SCE, Scene);
+    ID_PRV_CASE(ID_SCR, bScreen);
+    ID_PRV_CASE(ID_AC, bAction);
 #undef ID_PRV_CASE
     default:
       break;

source/blender/editors/include/ED_render.h

@@ -82,6 +82,8 @@ typedef enum ePreviewRenderMethod {
 void ED_preview_ensure_dbase(void);
 void ED_preview_free_dbase(void);
 
+bool ED_preview_id_is_supported(const struct ID *id);
+
 void ED_preview_shader_job(const struct bContext *C,
                            void *owner,
                            struct ID *id,

source/blender/editors/interface/interface_icons.c

@@ -2037,11 +2037,14 @@ void UI_icon_render_id(
     const bContext *C, Scene *scene, ID *id, const enum eIconSizes size, const bool use_job)
 {
   PreviewImage *pi = BKE_previewimg_id_ensure(id);
-
   if (pi == NULL) {
     return;
   }
 
+  if (!ED_preview_id_is_supported(id)) {
+    return;
+  }
+
   ui_id_preview_image_render_size(C, scene, id, pi, size, use_job);
 }
 

source/blender/editors/render/render_preview.c

@@ -786,6 +786,11 @@ struct ObjectPreviewData {
   int sizey;
 };
 
+static bool object_preview_is_type_supported(const Object *ob)
+{
+  return OB_TYPE_IS_GEOMETRY(ob->type);
+}
+
 static Object *object_preview_camera_create(Main *preview_main,
                                             ViewLayer *view_layer,
                                             Object *preview_object)
@@ -1658,9 +1663,12 @@ static void icon_preview_startjob_all_sizes(void *customdata,
     if (ip->id != NULL) {
       switch (GS(ip->id->name)) {
         case ID_OB:
-          /* Much simpler than the ShaderPreview mess used for other ID types. */
-          object_preview_render(ip, cur_size);
-          continue;
+          if (object_preview_is_type_supported((Object *)ip->id)) {
+            /* Much simpler than the ShaderPreview mess used for other ID types. */
+            object_preview_render(ip, cur_size);
+            continue;
+          }
+          break;
         case ID_AC:
           action_preview_render(ip, cur_size);
           continue;
@@ -1749,6 +1757,17 @@ static void icon_preview_free(void *customdata)
   MEM_freeN(ip);
 }
 
+/**
+ * Check if \a id is supported by the automatic preview render.
+ */
+bool ED_preview_id_is_supported(const ID *id)
+{
+  if (GS(id->name) == ID_OB) {
+    return object_preview_is_type_supported((const Object *)id);
+  }
+  return BKE_previewimg_id_get_p(id) != NULL;
+}
+
 void ED_preview_icon_render(
     const bContext *C, Scene *scene, ID *id, uint *rect, int sizex, int sizey)
 {