segmentation fault when function from MainWindow is running

[darkrptyp]

void MainWindow::change_gui_after_downloading(int value){
    if(value){                  
               ui->actionPc->setEnabled(false); //error here
    }
    else{
        ui->actionPc->setEnabled(true); //error here
   }
}

void MainWindow::change_gui_after_downloading(int value){
    if(value){                  
               ui->actionPc->setEnabled(false); //error here
    }
    else{
        ui->actionPc->setEnabled(true); //error here
   }
}

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and method in another class

void download::access_to_gui(){
     MainWindow* pointer
    if(this->download_value) 
        pointer->change_gui_after_downloading(1);
    else
        pointer->change_gui_after_downloading(0);
}

void download::access_to_gui(){
     MainWindow* pointer
    if(this->download_value) 
        pointer->change_gui_after_downloading(1);
    else
        pointer->change_gui_after_downloading(0);
}

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When I create new object for MainWindow class in download class I get "QWidget: Must construct a QApplication before a QPaintDevice", when I create pointer for MainWindow I get "Segmentation fault" in change_gui_after_downloading(int) function.
When I run change_gui_after_downloading(int) directly in MainWindow its working.

[Lykurg]

You need to pass a pointer of your actual main window to the class download. Otherwise you deal with 0 pointers or with different main windows. (Basic C++ and has nothing to do with Qt!)

Alternatively (when download is created in MainWindow) you can use signal and slots.