AutoCAD .NET :: How To Using Modeless Form

I want to AutoCAD do not lock it's space, when my Form is shown on display. But I have an exception when I use modeless dialog.

public void MyCommand ()
{
//-------------------------------------------------
// Using this code, my program works fine, but locks modal space of AutoCAD
System.Windows.Forms.Application.EnableVisualStyles ();
System.Windows.Forms.Application.Run (new  Form1 ());

  // I want to use this instruction to do not lock modal space
  // Autodesk.AutoCAD.ApplicationServices.Application.ShowModelessDialog (new Form1 ());
  //------------------------------------------------- 
}

If I use modeless dialog I can read and write from acad prompt and create block; BUT I can't create BlockReference. 

I get an exception "eLockViolation" in the next code instructions:

//------------------------------------------------------------------------------------------------------------------------------

public void CreateBlockReference (string strBlockName, Point3d Origin) {
using( Transaction t = db.TransactionManager.StartTransaction () )  {
BlockTable btTable = (BlockTable)t.GetObject (db.BlockTableId, OpenMode.ForRead);  // <--- EXCEPTION

[Code] ..........