Do not throw exception when the debugger encounters thread which we do not know about.

src/AddIns/Debugger/Debugger.Core/ManagedCallback.cs

@@ -74,7 +74,7 @@ namespace Debugger
 		void EnterCallback(PausedReason pausedReason, string name, ICorDebugThread pThread)
 		{
 			EnterCallback(pausedReason, name, pThread.GetProcess());
-			process.SelectedThread = process.Threads[pThread];
+			process.SelectedThread = process.GetThread(pThread);
 		}
 		
 		void ExitCallback()
@@ -128,7 +128,7 @@ namespace Debugger
 		{
 			EnterCallback(PausedReason.StepComplete, "StepComplete (" + reason.ToString() + ")", pThread);
 			
-			Thread thread = process.Threads[pThread];
+			Thread thread = process.GetThread(pThread);
 			Stepper stepper = process.GetStepper(pStepper);
 			
 			StackFrame currentStackFrame = process.SelectedThread.MostRecentStackFrame;
@@ -378,7 +378,7 @@ namespace Debugger
 				
 				EnterCallback(PausedReason.Other, "NameChange: pThread", pThread);
 				
-				Thread thread = process.Threads[pThread];
+				Thread thread = process.GetThread(pThread);
 				thread.NotifyNameChanged();
 				
 				ExitCallback();
@@ -445,7 +445,7 @@ namespace Debugger
 			if (process.Threads.Contains(pThread)) {
 				EnterCallback(PausedReason.Other, "ExitThread " + pThread.GetID(), pThread);
 				
-				process.Threads[pThread].NotifyExited();
+				process.GetThread(pThread).NotifyExited();
 			} else {
 				EnterCallback(PausedReason.Other, "ExitThread " + pThread.GetID(), process.CorProcess);
 				

src/AddIns/Debugger/Debugger.Core/Process.cs

@@ -233,6 +233,18 @@ namespace Debugger
 			return written;
 		}
 		
+		internal Thread GetThread(ICorDebugThread corThread)
+		{
+			foreach(Thread thread in this.Threads) {
+				if (thread.CorThread == corThread) {
+					return thread;
+				}
+			}
+			Thread t = new Thread(this, corThread);
+			this.Threads.Add(t);
+			return t;
+		}
+		
 		#region Exceptions
 		
 		public event EventHandler<ExceptionEventArgs> ExceptionThrown;

src/AddIns/Debugger/Debugger.Core/ThreadCollection.cs

@@ -38,16 +38,5 @@ namespace Debugger
 			}
 			return false;
 		}
-		
-		internal Thread this[ICorDebugThread corThread] {
-			get {
-				foreach(Thread thread in this) {
-					if (thread.CorThread == corThread) {
-						return thread;
-					}
-				}
-				throw new DebuggerException("Thread is not in collection");
-			}
-		}
 	}
 }