Add support for PreferentialStudy in study type validation

.vscode/settings.json

@@ -8,6 +8,7 @@
         "HITL",
         "hovertemplate",
         "Hyperband",
+        "isinstance",
         "libpython",
         "rhinocode",
         "showlegend",

Optuna/Study/StudyWrapper.cs

@@ -28,10 +28,11 @@ public StudyWrapper(dynamic study)
             PyInstance = study;
 
             dynamic optuna = Py.Import("optuna");
+            dynamic dashboard = Py.Import("optuna_dashboard");
             dynamic py = Py.Import("builtins");
-            if (!py.isinstance(study, optuna.study.Study))
+            if (!py.isinstance(study, optuna.study.Study) && !py.isinstance(study, dashboard.preferential.PreferentialStudy))
             {
-                throw new ArgumentException("study must be an instance of optuna.study.Study.");
+                throw new ArgumentException("study must be an instance of optuna.study.Study or optuna_dashboard.preferential.PreferentialStudy.");
             }
         }