Tanks can be pretty annoying to work with when that message pops up. As I see it, if a tank becomes full while there is an outgoing flow, the inflow should simply be adjusted to match the outflow. If you continue the simulation or disable errors messages, this is exactly what happens and therefore the error is not really needed in my opinion.
There is a workaround, involving two sensor controls:
The sensor (SensorStart event) will be triggered before it is accounted for as being full (TankFull event), so the error message will not pop-up.
You could apply the same workaround for the empty state (sensor position 0%), but now lock the exit. Again you need a second sensor slightly above to open the exit again. This kind of control is called a Schmitt trigger, just for your information.