Race condition resulting in unhandled exception and VS crash

[martinky]

In large solutions (thousands of indexed files) where the indexing takes quite long, the user may do this:

1. Invoke the plugin window, type a few letters,
2. close the window,
3. Invoke the window again, type more letters
4. --> crash

This is what happens internally:

1. On the first invoke a full index of the solution is started in a background thread. This indexing takes long enough time that it still runs when the plugin window is invoked again.
2. On the second invoke, another parallel index is started (the first one still runs).
3. When the first index finishes, it starts a search over the indexed data.
4. While the search executes, the second index finishes and rewrites the index collection on which the search query currently executes, resulting in a race condition.