反复调用函数GetTPData，偶尔会弹出错误：Project Project1.exe raised exception class 

EAccessViolation with message 'Access violation at address 00C3099A. Read of address 

FFFFFFFF'. Process stopped. Use Step or Run to continue. 到底是什么原因呢？

function TfrmMain.getBitStr(aHandle:HWND;ileft,iTop,iWidth,iHeight:integer;IsYDData:boolean):TStringList;
var
  FullscreenCanvas:TCanvas;
  dc:HDC;
  i,j,h:integer;
  c:TColor;
  Str,strline:string;
begin
  Result:=TstringList.Create ;
  dc:= GetDC (aHandle);  
  FullscreenCanvas := TCanvas.Create; 
  FullscreenCanvas.Handle := dc;
  if IsYDData then
    c:=clLime   
  else c:=fullscreenCanvas.Pixels[1,1]; 
  Str:=' ';
  for i :=1 to iHeight do
  begin  //i
    strline:='';
    for j :=1 to iWidth do
    begin   //j
        if c<>FullscreenCanvas.Pixels[iLeft+j,iTop +i] then
        begin   //if
          c:=FullscreenCanvas.Pixels[iLeft+j,iTop +i];
          if str=' ' then
            str:='1'
          else
            str:=' ';
        end;  //if
        strLine:=strLine+str;
    end;  //j
    Result.Add(strLine);  
  end; //i
//ReleaseDC(aHandle,dc);   此句用不用都不影响弹出错误提示
//ReleaseDC(aHandle,FullscreenCanvas.Handle);  此句用不用都不影响弹出错误提示
  FullscreenCanvas.Free;    
end;

\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

function GetTPData(AHwnd:HWND;T:integer):boolean; 
var        
  i,j,m,n:integer;
  strbit:TStringList;
  TempTP:string;
  Ok:boolean;
begin
  m:=455;
  Ok:=False;
  for i:=0 to 3 do 
  begin    //i
    for j:=0 to 3 do
    begin   //j
      strbit:=frmMain.getBitStr(AHwnd,m+j,T+i,34,16,False); 
      TempTP:='';
      if Trim(strbit.Strings[0])<>'' then  
      begin   //if
        for n:=0 to strbit.Count-1 do
        begin
          TempTP:=TempTP+strbit.Strings[n];
        end;
        Ok:=(TempTP=frmMain.TPdat.Strings[10]);  
      end;   //if
      strbit.Free;   
      if Ok then break; 
    end;   //j
    if Ok then break;
  end;   //i
  Result:=Ok;
end;