fix exception that was previously triggered when trying to insert after loading a parsed tree, by making sure that the arrowX and arrowY fields are clamped by the new canvas width and height

fcore/app-with.sml

@@ -1000,7 +1000,7 @@ struct
       }
     end
 
-  fun useLayerTree (app: app_type, layerTree, canvasWidth, canvasHeight) :
+  fun parsedLayerTree (app: app_type, layerTree, canvasWidth, canvasHeight) :
     app_type =
     let
       val
@@ -1028,6 +1028,11 @@ struct
         , layerTree = _
         , modalNum
         } = app
+
+      val arrowX =
+        if canvasWidth = 0 then 0 else Int.min (canvasWidth - 1, arrowX)
+      val arrowY =
+        if canvasHeight = 0 then 0 else Int.min (canvasHeight - 1, arrowY)
     in
       { mode = mode
       , mouseX = mouseX

fcore/normal-mode.sml

@@ -337,7 +337,7 @@ struct
   fun useLayers (model, layerTree, canvasWidth, canvasHeight) =
     let
       val model =
-        AppWith.layerTree (model, layerTree, canvasWidth, canvasHeight)
+        AppWith.parsedLayerTree (model, layerTree, canvasWidth, canvasHeight)
     in
       getDrawMessage model
     end