fixed a NPE when opening non-xtext file extensions suffixed with "xt".(cherry picked from commit 6b653c90a76108982c97dcc3ec5bce7764ff10bb)
improved messaged in case a file could not be loaded(cherry picked from commit b13fb8788def2b608ebd205e44a2dd290a4f2208)
fixed issues with test setups(cherry picked from commit b6d545f632c542d200f6f92c8cdf92b4912c2132)
simplified dependencies(cherry picked from commit f25b4001ba3790117bd8ec54a8a110354603f07f)
fixed a potential NPE