VS2019 Fails to Refresh Tasks on Open

# Summary

When opening a cake enabled solution for the 1st time in VS2019 the task explorer isn't populated.

# Repro Steps

1. Close Visual Studio 2019
2. Open Visual Studio 2019
3. Open a solution with a cake file, mine is a solution item
4. Open task runner explorer window

Expected: to see tasks from cake file
Actual: empty set of tasks

# Workaround

Clicking the refresh button in the top left of task runner explorer fixes it and shows all tasks for the duration of the session.

Reproduce the issue in Visual Studio 2019 using a solution with a Cake file, then open Task Runner Explorer immediately after starting Visual Studio. Begin by tracing the extension's solution-open and task-refresh entry points. Done means Cake tasks appear automatically on the first opening, without clicking the refresh button.

Written by the indexing model from the issue text.