Modification of Utility Method to Support Multi-Threading

I have been given a application to multi-thread and this has been done. All message boxes in this application are correctly called by a single static method in a utility class. This method is:

public static void ErrMsg(String strMsg, Exception ex = null)
{
    ...
    if (ex == null)
        MessageBox.Show(strMsg, "MyApp", MessageBoxButtons.OK, MessageBoxIcon.Warning);
    ...
}

Clearly this does not provide the IWin32Window owner and this is now causing me a problem when I invoke an error message from a background thread-pool thread using this method. The problem is a known one of the message box showing behind my main form. See:

1. Popping a MessageBox for the main app with Backgroundworker in WPF

et al. I could pass in the SynchronisationContext of the current thread to ErrMsg and do

synchronizationContext.Send(callback => 
    MessageBox.Show("Some error message for the user"), null);

But there are 700+ calls to this method, the majority of which are on the UI thread and do not cause a problem. My question is: how can I amend the ErrMsg method so that my message box appears in front reguardless of the current SynchronisationContex and without having to ammend all 700+ calls to the method?

Thanks for your time.

Edit. @Dmitry's idea was great. However, if the active form does not have focus or is an MdiChild form then the Form.ActiveForm will return null. To get around this I use Application.OpenForms.Cast<Form>().Last() to get the last active form, beit MdiChild or whatever. The code is now...

Form activeForm = Application.OpenForms.Cast<Form>().Last();
if (ex == null)
{
    activeForm.Invoke(new ShowErrMsg(text => 
        MessageBox.Show(activeForm, 
            text, 
            "UserCost",
            MessageBoxButtons.OK, 
            MessageBoxIcon.Warning)), 
        strMsg);
}