Fix being unable to load NBT in web viewer

lib/data.dart

@@ -358,6 +358,7 @@ class SessionData {
     }
 
     ListTag poses = ct.get("pos")! as ListTag;
+    positions.clear();
     for (var pos in poses.value) {
       positions.add(await SmallPosition.fromNBT(pos.asCompoundTag()));
     }
@@ -461,6 +462,9 @@ class SessionData {
         // Convert this to a CompoundTag
         CompoundTag ct = await NbtIo.readFromStream(lst) as CompoundTag;
         _deserialize(ct);
+
+        IsReadOnly = true;
+        isWeb = true;
         return true;
       } else
         return false;