Uncaught Error: [] operator not supported for strings in ... Ga_time_table\classes\fitnessFixedConstraint.php:124 #1

when i trying to generate the schedule, this error appear :

Fatal error: Uncaught Error: [] operator not supported for strings in C:...\Ga_time_table\classes\fitnessFixedConstraint.php:124 Stack trace: #0 C:...\Ga_time_table\tTable_gen.php(196): classes\fitnessFixedConstraint->clashingClasses('#group') #1 C:...\Ga_time_table\tTable_gen.php(155): index->calculateFitness(Array, Array, 21) #2 C:...\Ga_time_table\tTable_gen.php(233): index->initialise_schedule() #3 {main} thrown in C:...\Ga_time_table\classes\fitnessFixedConstraint.php on line 124

Could somebody please give me a hint what I'm doing wrong?

thank you