The calling thread must be STA, because many UI components require this

I'm try to focus on browse button through following code

if (!System.IO.Directory.Exists(MergeFilePath))
            {
                MessageBoxResult messageBoxResult = System.Windows.MessageBox.Show("Please select Destination Folder", "Select Destination Folder", System.Windows.MessageBoxButton.OK);
                btnChangeDestinationPath.Focus();
            }

  <Button Name="btnChangeDestinationPath" Width="75" Click="btnChangeDestinationPath_Click">Change</Button>

I'm getting following error:

"The calling thread must be STA, because many UI components require this."

{Function evaluation disabled because a previous function evaluation timed out. You must continue execution to reenable function evaluation.}