Columbia REIT, a landlord of office space used by Twitter in San Francisco, has filed a suit over unpaid rent against the social media company. “The litigation is a normal and expected action to occur when a tenant has a lease and does not pay rent required by a valid contract with the landlord,” said Carl Tobias, a law professor at the University of Richmond. “These types of disputes often settle without provoking litigation, so as to avoid litigation costs and bad publicity.”