Powershell.Create returns null on debugging

On running my below code, I get error on Powershell.create as below, has anyone seen this error before, I have Micosoft.Powershell.Sdk installled. What is wrong?? also I dont have InvokeAsync method in my powershell methods? why is that

Code:

public async Task<string> RunScript( string contentScript, Dictionary<string, EntityProperty> parameters)
        {
            List<string> parameterList = new List<string>();
            foreach( var item in parameters )
            {
                parameterList.Add( item.Value.ToString() );
            }
            using( PowerShell ps = PowerShell.Create() )
            {
                ps.AddScript( contentScript );
                ps.AddParameters( parameters );
                
                IAsyncResult async = ps.BeginInvoke();
                StringBuilder stringBuilder = new StringBuilder();
                foreach( PSObject result in ps.EndInvoke( async ) )
                {
                    stringBuilder.AppendLine( result.ToString() );
                }
                return stringBuilder.ToString();
            }
        }

Error:

System.NullReferenceException: 'Object reference not set to an instance of an object.' 5__4 was null.