I don't know why this happened, but there are two frame names, Frame1 and Frame1_1 in your form definition. In the unit1.pas and unit1.lfm file, there exists Frame1_1. But in the property inspector of your form, the button's action is TFrame1.FileOpen1 (while the frame name is Frame1_1, in the property inspector).
I tried to change action name of the button to Frame1_1.FileOpen1, but it was not successful. Then I tried to change the name of Frame1_1 to Frame1 in the property inspector, which was not successful either.
So I changed the names (Frame1_1 --> Frame1) directly in unit1.lfm file and unit1.pas, and then it seems to work correctly.