Fix possible NRE (issue #574).
Fix possible NRE (issue #574).

Ensure that 'rootElement' is non-null when accessing its 'ChildNodes' property.

chipitsine/msbuild,nikson/msbuild,chipitsine/msbuild,rainersigwald/msbuild,nikson/msbuild,nikson/msbuild,mono/msbuild,cdmihai/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,cdmihai/msbuild,chipitsine/msbuild,rainersigwald/msbuild,Microsoft/msbuild,rainersigwald/msbuild,AndyGerlicher/msbuild,jeffkl/msbuild,cdmihai/msbuild,sean-gilliam/msbuild,chipitsine/msbuild,sean-gilliam/msbuild,AndyGerlicher/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,nikson/msbuild,mono/msbuild,rainersigwald/msbuild,mono/msbuild,mono/msbuild,cdmihai/msbuild,Microsoft/msbuild,sean-gilliam/msbuild,Microsoft/msbuild,jeffkl/msbuild,jeffkl/msbuild,AndyGerlicher/msbuild