Fix Null Reference Exception (#468)

- Added a null check when accessing the token result which can be null
if there is an underlying MSAL or tokenprovider error.
- related to issue #455

CredentialProvider.Microsoft/CredentialProviders/Vsts/VstsCredentialProvider.cs

@@ -136,7 +136,7 @@ public override async Task<GetAuthenticationCredentialsResponse> HandleRequestAs
                 try
                 {
                     var result = await tokenProvider.GetTokenAsync(tokenRequest, cancellationToken);
-                    bearerToken = result.AccessToken;
+                    bearerToken = result?.AccessToken;
                 }
                 catch (Exception ex)
                 {