HtmlEditorExtender align justify not working as expected and unable to upload images

I am trying to implement the Ajax control toolkit HtmlEditorExtender in a web form but I am getting the following error:

0x800a139e - JavaScript runtime error: Sys.ArgumentException: Cannot deserialize empty string

I do not know how or why this error is happening. my attempts to search on the internet have been unsuccessful. Any advice you could give would be greatly appreciated

Also the align buttons don't seem to work. when I switch it to code view it displays the align property but does not align the text to the right or in the centre not sure why.

My aspx:

<ajaxToolkit:HtmlEditorExtender EnableSanitization="true" ID="HtmlEditorExtender1" OnImageUploadComplete="HtmlEditorExtender1_ImageUploadComplete" DisplaySourceTab="true" TargetControlID="txtPost" runat="server"><Toolbar><ajaxToolkit:Undo /><ajaxToolkit:Redo /><ajaxToolkit:Bold /><ajaxToolkit:Italic /><ajaxToolkit:Underline /><ajaxToolkit:StrikeThrough /><ajaxToolkit:Subscript /><ajaxToolkit:Superscript /><ajaxToolkit:InsertOrderedList /><ajaxToolkit:InsertUnorderedList /><ajaxToolkit:CreateLink /><ajaxToolkit:UnLink /><ajaxToolkit:RemoveFormat /><ajaxToolkit:BackgroundColorSelector /><ajaxToolkit:ForeColorSelector /><ajaxToolkit:FontSizeSelector /><ajaxToolkit:Indent /><ajaxToolkit:Outdent /><ajaxToolkit:InsertImage />       </Toolbar></ajaxToolkit:HtmlEditorExtender><asp:TextBox ID="txtPost" runat="server" BackColor="White" CssClass="form-control" TextMode="MultiLine" Rows="30"></asp:TextBox>

My code behind:

protected void HtmlEditorExtender1_ImageUploadComplete(object sender, AjaxControlToolkit.AjaxFileUploadEventArgs e)
{       
    string fullPath = "~/images/postAssets/" + e.FileName;
         // Save upload file to the file system
       HtmlEditorExtender1.AjaxFileUpload.SaveAs(MapPath(fullPath));
       e.PostedUrl = Page.ResolveUrl(fullPath);
}

As I said I have no idea why this is happening. I've never used the control before so don't really know what I'm doing