Ошибка : QWidget: Must construct a QApplication before a QWidget

from pyqtgraph.Qt import QtCore, QtGui
import pyqtgraph as pg
import pyqtgraph.opengl as gl
from PyQt5 import QtWidgets

from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *  

import numpy as np
from stl import mesh

from pathlib import Path


        
class MyWindow(QMainWindow):
    def __init__(self):
        super(MyWindow, self).__init__()
        self.setGeometry(0, 0, 700, 900) 
        self.setAcceptDrops(True)
        
        self.initUI()
        
        self.currentSTL = None
        self.lastDir = None
        
        self.droppedFilename = None
    
    def initUI(self):
        centerWidget = QWidget()
        self.setCentralWidget(centerWidget)
        
        layout = QVBoxLayout()
        centerWidget.setLayout(layout)
        
        self.viewer = gl.GLViewWidget()
        layout.addWidget(self.viewer, 1)
        
        self.viewer.setWindowTitle('STL Viewer')
        self.viewer.setCameraPosition(distance=40)
        
        g = gl.GLGridItem()
        g.setSize(200, 200)
        g.setSpacing(5, 5)
        self.viewer.addItem(g)

        btn = QPushButton(text="Load STL")
        btn.clicked.connect(self.showDialog)
        btn.setFont(QFont("Ricty Diminished", 14))
        layout.addWidget(btn)
            
    def showDialog(self):
        directory = Path("")
        if self.lastDir:
            directory = self.lastDir
        fname = QFileDialog.getOpenFileName(self, "Open file", str(directory), "STL (*.stl)")
        if fname[0]:
            self.showSTL(fname[0])
            self.lastDir = Path(fname[0]).parent
            
    def showSTL(self, filename):
        if self.currentSTL:
            self.viewer.removeItem(self.currentSTL)

        points, faces = self.loadSTL(filename)
        meshdata = gl.MeshData(vertexes=points, faces=faces)
        mesh = gl.GLMeshItem(meshdata=meshdata, smooth=True, drawFaces=False, drawEdges=True, edgeColor=(0, 1, 0, 1))
        self.viewer.addItem(mesh)
        
        self.currentSTL = mesh
        
    def loadSTL(self, filename):
        m = mesh.Mesh.from_file(filename)
        shape = m.points.shape
        points = m.points.reshape(-1, 3)
        faces = np.arange(points.shape[0]).reshape(-1, 3)
        return points, faces

    def dragEnterEvent(self, e):
        print("enter")
        mimeData = e.mimeData()
        mimeList = mimeData.formats()
        filename = None
        
        if "text/uri-list" in mimeList:
            filename = mimeData.data("text/uri-list")
            filename = str(filename, encoding="utf-8")
            filename = filename.replace("file:///", "").replace("\r\n", "").replace("%20", " ")
            filename = Path(filename)
            
        if filename.exists() and filename.suffix == ".stl":
            e.accept()
            self.droppedFilename = filename
        else:
            e.ignore()
            self.droppedFilename = None
        
    def dropEvent(self, e):
        if self.droppedFilename:
            self.showSTL(self.droppedFilename)

if __name__ == '__main__':
    app = QtWidgets.QApplication([])
    window = MyWindow()
    window.show()
    app.exec_()