The Project Navigator parser is expecting the source HDL file to be "well formed" when it comes to component instantiations. In cases where the design files are being edited, this might not always be the case. In some cases where syntax errors or illegal constructs are introduced into the top level source file, the hierarchy parser is not able to interpret the file correctly and returns a bad pointer, which in turn causes the system to crash.
This problem has been fixed in the latest 10.1 Service Pack available at:
http://www.xilinx.com/support/download/index.htmThe first service pack containing the fix is 10.1 Service Pack 2
NOTE: The fix to this issue inserts code to "qualify" the cast prior to accessing the pointer. If the cast is not valid, the underlying code labels the design unit as being invalid and continues normal operations. This prevents Project Navigator from crashing, but does not highlight improper HDL code which might result in an invalid hierarchy. Check Syntax should be run on each source file to validate the proper HDL syntax.