Can anyone let me know where I'm going wrong?  Below is my Apex class. I get the following error when the VF page tries to call it. I'm sure it's because it can't convert String to a List, but I have no idea how to fix it. The email examples given by Salesforce always have a hardcoded email address.Thank you for any help!           

common.apex.runtime.impl.ExecutionException: Invalid conversion from runtime type String to List<String>

public with sharing class AttachmentUploadController

{
    public String EmailSubject { get;  set; }
    public String EmailBody { get;  set; }
    public String ToEmails{ get;  set; }
    
    public Attachment attachment 
    {
        get
        {
            if (attachment == null)
            attachment = new Attachment();
            return attachment;
        }
        set;
    }

    public PageReference sendEmail() 
    {
        String parentId = System.currentPagereference().getParameters().get('id');
        String[] ToAddresses = this.ToEmails.split(';');
        
        try
        {

            //Start: Send Email with Attachment
            Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();                    
                        
            mail.setToAddresses(ToAddresses);
            mail.setHTMLBody(this.EmailBody);
            mail.setSubject(this.EmailSubject);         

            //Set email file attachments
            List<Messaging.Emailfileattachment> fileAttachments = new List<Messaging.Emailfileattachment>();

            // Add to attachment file list
            Messaging.Emailfileattachment efa = new Messaging.Emailfileattachment();
            efa.setFileName(attachment.Name);
            efa.setBody(attachment.Body);
            fileAttachments.add(efa);

            //create attachment for object

            mail.setFileAttachments(fileAttachments);

            //Send email
            Messaging.sendEmail(new Messaging.SingleEmailMessage[] { mail });
            //END : Send Email with Attachment

            PageReference page = new PageReference('/' + parentId);
            return page;
        } 
        catch (DMLException e) 
        {
            ApexPages.addMessage(new ApexPages.message(ApexPages.severity.ERROR,'Email Not Sent'));
            return null;
        }
        finally
        {
            attachment = new Attachment(); 
        }
    }
  }