BELFAST—Northern Ireland’s power-sharing government put aside factional differences on Thursday to appeal for calm after more than a week of nightly violence partly fuelled by frustration among pro-British unionists over post-Brexit trade barriers. Hundreds of youths in the British province’s capital Belfast set a hijacked bus on fire and attacked police with stones on Wednesday in scenes reviving memories of decades of sectarian strife that claimed some 3,600 lives prior to a 1998 peace deal. The latest violence has injured 55 police officers and seen boys as young as 13 and 14 arrested on rioting charges. “We are gravely concerned by the scenes we have all witnessed on our streets,” said the compulsory coalition, led by rival pro-Irish Catholic nationalists and pro-British Protestant unionists. “While our political positions are very different on many issues, we are all united in our support for law and order and we collectively state our …