delphi 的问题~~~~~~~~~~

我在CreateForm里面调用这个方法，就给报Access violation的错误。。求解救。
Form1->Images->Canvas->StretchDraw(TRect(0, 0, 480, 320), gBufs);这句在就报~~~

void PaintAlls()
{
    gBufs->SetSize(gMapData.bgImageDis->Width, gMapData.bgImageDis->Height);

    TCanvas* canvas = gBufs->Canvas;
    TRect rect( 0, 0, gBufs->Width, gBufs->Height );
    canvas->Brush->Color = 0;
    canvas->FillRect(rect);
    canvas->Draw( gMapData.x, gMapData.y, gMapData.bgImageDis );

    vector<MapObjects>::iterator itor = gMapData.objects.begin();
    while (itor != gMapData.objects.end()) {
        itor->Draw(canvas, gMapData.x, gMapData.y);
        itor++;
    }

    // 框框
    gMapData.smallK.Draw(canvas, FormMain->Image1->ClientWidth,
    FormMain->Image1->ClientHeight);

    Form1->Images->Canvas->StretchDraw(TRect(0, 0, 480, 320), gBufs);
};