The SDF provider throws an unessessary exception

[badreddinekaroui]

The SDF throws an exception when it does not find an Rtree node. This is an unesessary exception since it may just be a case of race condition where the node is not flushed yet by the writer. This issue happened in a rare condition where a writer is saving a huge SDF file(>5millions point features)while a spatial query on a separate thread is continuously executing and fetching the result. Eventually the writer finishes and the Rtree is fine.