Delphi first chance exception

What does this error mean?

first chance exception at $0068C7AA. Exception class $0000005 with message 'access violation at 0x0068c7aa: write of address 0x00000000'. Process Project1.exe (5720)

The error points to this line of code:

ArrMain[i] := StrToInt(StringGrid1.Cells[1,i]) + StrToInt(StringGrid1.Cells[3,i]);