The only thing you should be worried about with an electric heater in this location is if the heater fails and leaks to lower floors or if the heater's pressure relief valve opens to let water escape.

If the heater is located in a catch basin/drip pan with a drain to your sewer lines, you shouldn't have any worries.

But another quirk of your townhome is that because it exceeds 3 stories, it is not regulated by residential building codes, but rather commercial building codes.

You'd have to call your local code comemrcial code enforcement office to determine if an electric heater in a closet is a commercial code violation.