After adding your solution error is gone but I can see same item twice in Content editor. Item which are configured with Unicorn configuration.

Please Update Unicorn.Dataprovider.config for 10.1 with below.

<databases>
            <database id="master">
                <dataProviders>
                    <dataProvider>
                        <param desc="headProvider">
                            <dataProvider ref="dataProviders/main" param1="$(id)">
                                <patch:attribute name="ref">dataProviders/unicorn</patch:attribute>
                            </dataProvider>
                        </param>
                    </dataProvider>
                </dataProviders>
            </database>
            <database id="core">
                <dataProviders>
                    <dataProvider>
                        <param desc="headProvider">
                            <dataProvider ref="dataProviders/main" param1="$(id)">
                                <patch:attribute name="ref">dataProviders/unicorn</patch:attribute>
                            </dataProvider>
                        </param>
                    </dataProvider>
                </dataProviders>
            </database>
        </databases>