The `RenderFileAction` throws NullPointerException

[marek-parfianowicz]

originally reported in #122

2. The RenderFileAction NPE

java.lang.NullPointerException
    at ...reporters.html.RenderFileAction.call(RenderFileAction.java:111)
    at ...util.CloverExecutors$LoggingCallable.call(CloverExecutors.java:91)

This is still reproducible on 5.0.0. @JasonWThompson's diagnosis was correct. The details:

RenderFileAction holds its column/context state in two static fields:

protected static ThreadLocal<List<Column>> columnsTL;
protected static ThreadLocal<ContextSet> contextSetTL;

but their lifecycle is owned by HtmlReporter, which calls initThreadLocals() before submitting render tasks and resetThreadLocals() in a finally block — and that reset assigns the statics back to null.

I've reproduced this with a unit test on the 5.0.x branch.

Fix direction: make this state per-report rather than static, so each HtmlReporter owns its own lifecycle and one report's cleanup can't affect another's in-flight tasks. Additionally, the finally reset shouldn't run while tasks may still be executing.