Simon Tisdall: The decline in Tesla's share price has been particularly steep very recently. He says investors are worried that the distraction of owning and running Twitter means Elon Musk isn't giving his full attention to his car company. And there's a danger that that does in some sense damage Tesla's brand as well, he adds.Tisdall: But you have to accept that the problems of the car industry, the supply chain, fail-ups, getting access to chips will all affect Tesla too.