When Delivery Hero SE's stock plunged in their worst selloff on record last week, Chief Executive Officer Niklas Oestberg took to Twitter to apologize to shareholders. He also put his money where his mouth is. Oestberg, the co-founder of the Berlin-based food-delivery company, bought about 14 million euros ($16 million) worth of stock on Feb. 10, the day Delivery Hero saw more than 30% of its value wiped out, a regulatory filing showed on Tuesday.