Fix start node configuration for tree pickers

src/Umbraco.Web/PropertyEditors/ContentPickerConfiguration.cs

@@ -8,6 +8,6 @@ namespace Umbraco.Web.PropertyEditors
         public bool ShowOpenButton { get; set; }
 
         [ConfigurationField("startNodeId", "Start node", "treepicker")] // + config in configuration editor ctor
-        public int StartNodeId { get; set; } = -1; // default value is -1
+        public string StartNodeId { get; set; }
     }
-}
+}

src/Umbraco.Web/PropertyEditors/MediaPickerConfiguration.cs

@@ -17,6 +17,6 @@ namespace Umbraco.Web.PropertyEditors
         public bool DisableFolderSelect { get; set; }
 
         [ConfigurationField("startNodeId", "Start node", "mediapicker")]
-        public int StartNodeId { get; set; }
+        public string StartNodeId { get; set; }
     }
-}
+}

src/Umbraco.Web/PropertyEditors/MultiNodePickerConfigurationTreeSource.cs

@@ -15,6 +15,6 @@ namespace Umbraco.Web.PropertyEditors
         public string StartNodeQuery {get;set;}
 
         [JsonProperty("id")]
-        public int? StartNodeId {get;set;}
+        public string StartNodeId {get;set;}
     }
 }