Fix argument out of range exception when modify the registered NuGet package sources in Tools Options.

src/AddIns/Misc/PackageManagement/Project/Src/RegisteredPackageRepositories.cs

@@ -2,6 +2,7 @@
 // This code is distributed under the GNU LGPL (for details please see \doc\license.txt)
 
 using System;
+using System.Linq;
 using NuGet;
 
 namespace ICSharpCode.PackageManagement
@@ -54,8 +55,10 @@ namespace ICSharpCode.PackageManagement
 			get {
 				activePackageSource = options.ActivePackageSource;
 				if (activePackageSource == null) {
+					if (options.PackageSources.Any()) {
 						activePackageSource = options.PackageSources[0];
 					}
+				}
 				return activePackageSource;
 			}
 			set {

src/AddIns/Misc/PackageManagement/Test/Src/RegisteredPackageRepositoriesTests.cs

@@ -261,6 +261,18 @@ namespace PackageManagement.Tests
 			
 			Assert.AreEqual(expectedRepository, repository);
 		}
+		
+		[Test]
+		public void ActivePackageSource_AllPackageSourcesCleared_ReturnsNullAndDoesNotThrowArgumentOutOfRangeException()
+		{
+			CreateRegisteredPackageRepositories();
+			packageSourcesHelper.Options.ActivePackageSource = null;
+			registeredRepositories.PackageSources.Clear();
+			
+			var activePackageSource = registeredRepositories.ActivePackageSource;
+			
+			Assert.IsNull(activePackageSource);
+		}
 	}
 }