TypeError: unsupported operand type(s) for +: ‘float’ and ‘NoneType’

OS (e.g. Win10): win11
PsychoPy version (e.g. 1.84.x): v2022.2.5
Standard Standalone? (y/n) If not then what?:y
What are you trying to achieve?:
I wish to run an experiment with two condition files, one containing the chunk information and one containing all the trials for a single block.
But I found that when I printed out the variable ISI, I found that after the end of a block, that is, after the end of the last condition as a block, its value changed to None, I don’t know why?Any help would be greatly appreciated!!

Here is the line of code where my program went wrong:

Below are the printed results and errors: