Added an exit rule for abstract classes

ScriptableRenderPipeline/Core/Volume/Editor/VolumeEditor.cs

                 var script = (MonoScript)o;
                 var scriptType = script.GetClass();
 
-                if (!scriptType.IsSubclassOf(typeof(VolumeComponent)))
+                if (!scriptType.IsSubclassOf(typeof(VolumeComponent)) || scriptType.IsAbstract)
                     return false;
 
                 if (actualTarget.components.Exists(t => t.GetType() == scriptType))

ScriptableRenderPipeline/Core/Volume/VolumeManager.cs

                             .SelectMany(
                                 a => a.GetTypes()
                                 .Where(
-                                    t => t.IsSubclassOf(typeof(VolumeComponent))
+                                    t => t.IsSubclassOf(typeof(VolumeComponent)) && !t.IsAbstract
                                 )
                             );