Schema mismatch: Property X in class Y is nullable on one side and not on the other

My Realm app is failing to sync, and I’m getting the following error:

ERROR: Connection[1]: Session[1]: Failed to transform received changeset: Schema mismatch: Property ‘children’ in class ‘Folder’ is nullable on one side and not on the other.

In my app, Folders can have many, and belong to many Folders. I’m not sure what this error means by “nullable on one side and not on the other”?

MORE CONTEXT:

This is the config I’m using when opening a Realm connection:

const config = {
      schema: [Schema.Folder, Schema.Note],
      schemaVersion: 2.2,
      path: `${app.getPath('userData')}/${this.app.currentUser.id}/default.realm`,
      sync: {
        user: this.app.currentUser,
        partitionValue: this.app.currentUser.id,
        existingRealmFileBehavior: {
          type: "openImmediately"
        },
        newRealmFileBehavior: {
          type: "openImmediately"
        }
      }
    }

And as for the Schema.Folder and Schema.Notes it’s referencing, those are defined as this:

    const Folder = {
      name: "Folder",
      primaryKey: "_id",
      _partition: "string?",
      properties: {
        _id: "string",
        name: { type: "string", default: "Untitled Folder" },
        folder_id: { type: "string?", default: "" },
        is_collapsed: { type: "bool", default: true },
        children: {
          type: "list",
          objectType: "string"
        },
        notes: "Note[]",
        created_at: "date",
        updated_at: "date?",
        deleted_at: "date?",
      }
    }

    const Note = {
      name: "Note",
      "primaryKey": "_id",
      _partition: "string?",
      properties: {
        _id: "string", // uuid
        body: { type: "string?", default: "" },
        preview: "string?",
        is_pinned: { type: "bool", default: false },
        folder: {
          type: "linkingObjects",
          objectType: "Folder",
          property: "notes"
        },
        created_at: "date",
        updated_at: "date?",
        deleted_at: "date?",
        margins: { type: "bool", default: false },
      }
    }