Linq dynamic query problem - operator '&&' is incompatible with operand types 'String' and 'Boolean'

I have a method which generally forms my qeury. It looks like this:

        private static string PrepareWhereClause(string[] columns)
    {
        var query = $"({string.Join(" OR ", columns?.Select(c => $"{c} != null AND {c}.ToLower().Contains(@0)"))})";
        return query;
    }

This query is for the Where extension from Linq Dynamic as follows:

public static IQueryable<T> Search<T>(this IQueryable<T> queryable, string phrase, params string[] columns) =>
        queryable?.Where(PrepareWhereClause(columns), phrase.ToLower());

What is the query inside my object:

public class ResponsibilitiesWebDto
{
    public Guid Id { get; set; }
    public DictionaryBaseDto ResponsibilityType { get; set; }
    public UserForDetailedDto Employee { get; set; }
    public SupplierForDetailedDto Supplier { get; set; }
}

The parameters of PrepereWhereClause are fields of type string, which are in ResponsibilityType, Employee, and Supplier.

The query PrepereWhereClause produces is:

(ResponsibilityType.Name != null AND ResponsibilityType.Name.ToLower().Contains(@0) OR Employee.EmployeeId != null AND Employee.EmployeeId.ToLower().Contains(@0) OR Employee.Name != null AND Employee.Name.ToLower().Contains(@0) OR Employee.LastName != null AND Employee.LastName.ToLower().Contains(@0) OR Supplier.AccountNum != null AND Supplier.AccountNum.ToLower().Contains(@0) OR Supplier.Name != null AND Supplier.Name.ToLower().Contains(@0))

Everything is ok when all child elements are not empty. The problem is when some of them are empty. I was looking forward to this post about SO Dynamic Linq Core OrderBy Nullable Child Property and about the issue on github , I tried to use but it doesn't work :( I modified my query for someone:

     private static string PrepareWhereClause(string[] columns)
    {
        var query = $"({string.Join(" OR ", columns?.Select(c => $"{c} != null AND {c}.ToLower().Contains(@0)"))})";
        var query1 = $"({string.Join(" OR ", columns?.Select(c => "((" + ConvertToNullableNested(c) + $") AND {c}.ToLower().Contains(@0))"))})";
        var query2 = $"({string.Join(" OR ", columns?.Select(c => $"np({c} != null AND {c}.ToLower().Contains(@0)"))})";
        var query3 = $"({string.Join(" OR ", columns?.Select(c => $"(np({c}) AND {c}.ToLower().Contains(@0))"))})";
        var query4 = $"({string.Join(" OR ", columns?.Select(c => $"(np({c}) && {c}.ToLower().Contains(@0))"))})";
        return query4;
    }

The new query is:

((np(ResponsibilityType.Name) && ResponsibilityType.Name.ToLower().Contains(@0)) OR (np(Employee.EmployeeId) && Employee.EmployeeId.ToLower().Contains(@0)) OR (np(Employee.Name) && Employee.Name.ToLower().Contains(@0)) OR (np(Employee.LastName) && Employee.LastName.ToLower().Contains(@0)) OR (np(Supplier.AccountNum) && Supplier.AccountNum.ToLower().Contains(@0)) OR (np(Supplier.Name) && Supplier.Name.ToLower().Contains(@0)))

but I always get the error:

Operator '&&' incompatible with operand types 'String' and 'Boolean'

I tried changing && to AND but nothing changed. What am I doing wrong?

I changed the query like @orxanmuv write but I still get the error:

Query it:

(((np(ResponsibilityType.Name) == null or np(ResponsibilityType.Name)) AND ResponsibilityType.Name.ToLower().Contains(@0)) OR ((np(Employee.EmployeeId) == null or np(Employee.EmployeeId)) AND Employee.EmployeeId.ToLower().Contains(@0)) OR ((np(Employee.Name) == null or np(Employee.Name)) AND Employee.Name.ToLower().Contains(@0)) OR ((np(Employee.LastName) == null or np(Employee.LastName)) AND Employee.LastName.ToLower().Contains(@0)) OR ((np(Supplier.AccountNum) == null or np(Supplier.AccountNum)) AND Supplier.AccountNum.ToLower().Contains(@0)) OR ((np(Supplier.Name) == null or np(Supplier.Name)) AND Supplier.Name.ToLower().Contains(@0)))

Error: {"Operator 'or' is incompatible with operand types 'Boolean' and 'String'"}