I have a suggestion to you and need your helps to validte the crash problem is either in the RTree functionality or the other places, because we don't have the Delphi environment so please create another new sample application and reference the MapSuiteCore.dll, then copy the following code to your new sample application:
You can specify the upperLeftX, LowerRightY, LowerRightX, upperLeftY values by yourself, we just need to validate the rtree functionality. This is a simple sample and we use this function in MapSuiteGeocoder core code for reverse geocoding, also when you use this sample code above please through the COM methods to reference the MapSuiteCore.dll so that we can find out the crash reason.
RtreeSpatialIndex rtreeIndex = new RtreeSpatialIndex(Path.Combine(dataFilePath, "rt2.idx"));
Dictionary<string, RECTANGLE_D> streetsInsideZip = rtreeIndex.GetRecordByBoundingBoxIndexesContainingRectangle(upperLeftX, lowerRightY,lowerRightX, upperLeftY);