Description of problem:
I launched GNOME Builder and opened a meson-based project.

Version-Release number of selected component:
meson-0.45.0-1.fc28

Additional info:
reporter:       libreport-2.9.3
cmdline:        /usr/bin/python3 -s /usr/bin/meson introspect --tests
crash_function: join
exception_type: TypeError
executable:     /usr/bin/meson
kernel:         4.16.0-0.rc4.git0.1.fc28.x86_64
runlevel:       N 5
type:           Python3
uid:            13041

Truncated backtrace:
posixpath.py:78:join:TypeError: expected str, bytes or os.PathLike object, not NoneType

Traceback (most recent call last):
  File "/usr/bin/meson", line 26, in <module>
    sys.exit(main())
  File "/usr/bin/meson", line 23, in main
    return mesonmain.run(sys.argv[1:], launcher)
  File "/usr/lib/python3.6/site-packages/mesonbuild/mesonmain.py", line 293, in run
    return mintro.run(remaining_args)
  File "/usr/lib/python3.6/site-packages/mesonbuild/mintro.py", line 212, in run
    coredata = cdata.load(options.builddir)
  File "/usr/lib/python3.6/site-packages/mesonbuild/coredata.py", line 344, in load
    filename = os.path.join(build_dir, 'meson-private', 'coredata.dat')
  File "/usr/lib64/python3.6/posixpath.py", line 78, in join
    a = os.fspath(a)
TypeError: expected str, bytes or os.PathLike object, not NoneType

Local variables in innermost frame:
p: ('meson-private', 'coredata.dat')
a: None