Error generated by presentmodalviewcontroller in IOS.

When debugging the code, a controller is displayed in the model mode. The Code is as follows:

Systemstyleviewcontroller * s = [[systemstyleviewcontroller
Alloc] init];

Uinavigationcontroller * nav = [uinavigationcontroller
Alloc] initwithrootviewcontroller: S];

S. modalpresentationstyle =
Uimodalpresentationformsheet;

S. modaltransitionstyle =
Uimodaltransitionstylefliphorizontal;

[Self
Presentmodalviewcontroller: s animated: Yes];

The error is:

Application tried to present modally an active controller

If the uinavigationcontroller line is blocked, the program runs normally.

// Uinavigationcontroller * nav = [[uinavigationcontroller alloc] initwithrootviewcontroller: S];

The conjecture may be that after the navigationcontroller is generated, the system considers it as active.

But push does.

[Self. navigationcontroller
Pushviewcontroller: s animated: Yes];