Hello; Having learned the hard way many years ago, I now unplug heaters and allow them to sit in place a whle before removing them from a tank. If in a hurry i have a container that will not be affected by heat to place one in. I have melted a few things by laying a still hot heater on them.
When changing water I do not lower the water level lower than an inch or so above the heater element. I also do not stick a hot heater back into a tank. I have several heaters in a box that I put back into service from time to time I test them by plugging them in and running the thermostat up untill they come on to see if they still work. They will stay hot for a while after being unplugged so I usually have a glass bowl to put the in untill cold. I always put a cold heater into a tank.
Learning the hard way is likely going to be a part of the hobby. Good luck.