Mock a function that returns a Expression Tree in a class

Scenario:

I have a class with methods, in which one method returns a expression tree. How can I mock that method.

Code:

public Expression<Func<SpecFinderDataModel, bool>> BuildDynamicWhereClause(DataTableAjaxPostModel model)
    {
        var predicate = PredicateBuilder.New<SpecFinderDataModel>(true);
        if (_stringValidator.IsValid(model.search.value))
            predicate = _basicSearchService.DoSearch(model.search.value, predicate);
        var searchData = model.columns.Where(x => x.search.value != null);
        predicate = _advancedSearchService.DoSearch(model.isActive, searchData, predicate);
        return predicate;
    }

Here _advancedSearchService.DoSearch() this method returns a Expression<Func<SpecFinderDataModel, bool>>. I wrote a test which is given below

Test:

[TestMethod]
[TestCategory("BuildDynamicWhereClause")]
public void BuildDynamicWhereClauseTest()
{
    DataTableAjaxPostModel searchmodel = new DataTableAjaxPostModel()
    {
        columns = new List<Column>()
        {
            new Column() {
                data ="Status",
                orderable ="true",
                searchable ="true",
                search = new Search() {
                    regex = "false",
                    value ="TestStatus"
                }
            }
        },
        search = new Search()
    };

    IFinderBuildQueryFlow _finderBuildQueryFlow = new FinderBuildQueryFlow(
        _mockBasicSearchService.Object, _mockAdvancedSearchService.Object, _mockStringValidator.Object);

    var predicate = PredicateBuilder.New<SpecFinderDataModel>(true);
    _mockStringValidator.Setup(x => x.IsValid(searchmodel.search.value)).Returns(false);
    var searchData = searchmodel.columns.Where(x => x.search.value != null);
    _mockAdvancedSearchService.Setup(x => x.DoSearch(searchmodel.isActive, searchData, predicate).Compile()(model)).Returns(true);
    var test = _finderBuildQueryFlow.BuildDynamicWhereClause(searchmodel).Compile()(model);
}

Everything before _mockAdvancedSearchService.Setup(x => x.DoSearch(searchmodel.isActive, searchData, predicate).Compile()(model)).Returns(true); works fine. But I don't know how to mock advancedSearch.DoSearch() method.

Any advice would be really helpful. Thanks

Update #1:

This is the error when that line of code is executed

Update #2:

so here the predicate is returned as null.

and in the test I'm getting the error like this