Access violation in Delphi after successful run -

i have written program in delphi compute, display , save pascals' triangle user-defined number of rows. works fine (displays triangle, , allows me save it), except comes access violation @ end! here message:

access violation @ address 004031db in module 'project1.exe'. read of address 00000000.

i have 2d dynamic array in procedure release memory @ end (:= nil). why still giving me access violation? frustrating!

i searched archives answer not find appropriate answer. appreciated.

here code (i little hesitant there bit of code:

 procedure tform1.btnptclick(sender: tobject);  var    i, j, k, n, midcol: integer;    pt: array of array of integer;    row: string;  begin    k := strtoint(lblnumrows.text);    n := strtoint(lblnumrows.text);//# values in row = row number     try        //initiatlize array        setlength(pt, k, (n*2)-1);        := 0 k-1          j := 0 (n*2-1)            pt[i,j] := 0;         midcol := (n*2 div 2)-1;//midcol 0-based        := 0 k-1          begin            if (i = 0)              pt[i,midcol] := 1//first row gets 1 in middle column                else if = 1                  begin                    pt[i,midcol-1] := 1;                    pt[i,midcol+1] := 1; //first , last value in second = 1                  end             else //if other row               begin                  //middle column                 pt[i, midcol] := pt[i-1,midcol-1] + pt[i-1,midcol+1];                 //right triangle                 j := midcol+1 (n*2-1)                   begin                     if (pt[i-1, j-1]=1) then//if @ end of prev row                       begin                         pt[i,j] := 1;                         break;                       end                     else                       pt[i,j] := pt[i-1,j-1] + pt[i-1,j+1];                   end;                 //left triangle                 j := midcol-1 downto 0                   begin                     if (pt[i-1, j+1] = 1) //if @ end of prev row                       begin                         pt[i,j] := 1;                         break;                       end                     else                       pt[i,j] := pt[i-1,j-1] + pt[i-1,j+1];                   end;               end;          end;           //now add pyramid memo          application.processmessages;          := 0 k-1            begin              row := '';              j := 0 n*2-1                begin                  if (pt[i,j] = 0) row := row + ' '                  else row := row + inttostr(pt[i,j]);                end;              memo.lines.add(row);            end;          setlength(pt, 0, 0);    end;  end; 

read of address 00000000 

this indicates trying access memory using pointer nil. know why 1 need code. @ present have code, , can explain.

run program in debugger. enable debug dcus in case error raised in rtl/vcl code. ensure debugger configured break on exceptions. run program , trigger error. debugger show nil object being de-referenced. have work out why reference nil.

the code have added answer has buffer overrun explain problem. setlength incorrect , should read:

setlength(pt, k, n*2); 

your code writes memory out-of-bounds , corrupts heap. should ask compiler produce runtime checks on array bounds. enable compiler's range checking option. had done so, have found error yourself.

for worth not need try/finally block since compiler automatically insert hidden block. there's no need 2 when 1 suffices. dynamic array managed type memory disposed when variable leaves scope.