The stock price is tanking because there are roughly 7000 Model 3s in inventory that Tesla can't sell.
The difference between Model 3s produced and Model 3s delivered is just over 8000, but only 1000 were "in transit."
That means Tesla has 7000 Model 3s in inventory that they haven't been able to sell. That goes against the Model of "every car produced already has a buyer."
Furthermore, Tesla hasn't given any guidance on what to expect for 2019.
If demand does decline in the US and Tesla can't get EU homologation, the negative working capital becomes a huge problem.