Intellisense causes exceptions when COM objects are used

My C# code deals with COM objects. When I type in object name followed by '.' then SyntaxEditor tries to open Intellisense window and it fails. I used the following method to add COM Interop library to the project resolver:

m_dotNetProjectResolver.AddExternalReference(dllPath);

Here is the call stack:

NullReferenceException:
at ActiproSoftware.SyntaxEditor.Addons.DotNet.Dom.DotNetProjectResolver.a(IDomType A_0)
at ActiproSoftware.SyntaxEditor.Addons.DotNet.Dom.DotNetProjectResolver.GetExtensionMethods(IDomType contextType, String[] importedNamespaces, IDomType targetType, String name, DomBindingFlags flags)
at ActiproSoftware.SyntaxEditor.Addons.DotNet.Dom.DotNetProjectResolver.a(Hashtable A_0, DotNetContext A_1, IDomType A_2, DomBindingFlags A_3)
at ActiproSoftware.SyntaxEditor.Addons.DotNet.Dom.DotNetSyntaxLanguage.b(DotNetLanguage A_0, SyntaxEditor A_1, Boolean A_2)
at ActiproSoftware.SyntaxEditor.Addons.CSharp.CSharpSyntaxLanguage.ShowIntelliPromptMemberList(SyntaxEditor syntaxEditor)
at ActiproSoftware.SyntaxEditor.Addons.CSharp.CSharpSyntaxLanguage.OnSyntaxEditorKeyTyped(SyntaxEditor syntaxEditor, KeyTypedEventArgs e)
at ActiproSoftware.SyntaxEditor.SyntaxEditor.a(EditorView A_0, Keys A_1, Char A_2, Boolean A_3)
at ActiproSoftware.SyntaxEditor.SyntaxEditor.OnKeyPress(KeyPressEventArgs e)
at System.Windows.Forms.Control.ProcessKeyEventArgs(Message& m)
at System.Windows.Forms.Control.ProcessKeyMessage(Message& m)
at System.Windows.Forms.Control.WmKeyChar(Message& m)
at System.Windows.Forms.Control.WndProc(Message& m)
at System.Windows.Forms.ScrollableControl.WndProc(Message& m)
at ActiproSoftware.SyntaxEditor.SyntaxEditor.WndProc(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)