Fix a crash in gotoSrcLine

It appears that DocumentController::openDocument can return a null pointer.
So we should better test for it.
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Status Job ID Name Coverage
failed #56905
linux kf5-qt5 qt5.12


Name Stage Failure
linux kf5-qt5 qt5.12 Build
[ 29%] Built target kdevqmakecommon
[ 29%] Linking CXX shared module ../../bin/
[ 29%] Built target kdevflatpak
make: *** [Makefile:161: all] Error 2
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