D-Bus: Fix crash on properties without set accessors
D-Bus: Fix crash on properties without set accessors

flobrosch/vala-fixes,GNOME/vala,severinh/vala,GNOME/vala,frida/vala,jessevdk/vala,Distrotech/vala,tjol/vala-tjol,frida/vala,themiwi/vala,kamanashisroy/aroop,playya/vala,jessevdk/vala,kamanashisroy/aroop,GNOME/vala,Jc2k/vala,themiwi/vala,playya/vala,frida/vala,Jc2k/vala,tjol/vala-tjol,GNOME/vala,jessevdk/vala,themiwi/vala,frida/vala,frida/vala,Distrotech/vala,severinh/vala,xdave/pvala,flobrosch/vala-fixes,spidermonkdust/vala,flobrosch/vala-fixes,Distrotech/vala,severinh/vala,kamanashisroy/aroop,spidermonkdust/vala,rvu95/vala,tjol/vala-tjol,playya/vala,rvu95/vala,rvu95/vala,spidermonkdust/vala