Model is unsuitably constrained (i.e. there were still some variables left to be computed, and some unusable equations left over).
At the time of the error, the following variables were found:
 * Undefined: v in component Current_bNa
 * Undefined: E_Na in component Current_bNa
 * Undefined: i_b_Na in component Current_bNa
 * Defined: g_b_Na in component Current_bNa
The following equations couldn't be used:
 * Equation with no id on the apply or math element.