Load a shapefile using PyQGIS (QGIS 3.4) on ubuntu?

Question

I want to create a standalone application based on QGIS API. Here is my source code to load a shapefile:

import sys
from qgis.gui import *
from qgis.core import *
from qgis.core import QgsProject
from PyQt5.QtGui import *
from PyQt5.QtCore import Qt
from PyQt5.QtWidgets import QMainWindow, QApplication, QVBoxLayout, QWidget


class MapViewer(QMainWindow):
    def __init__(self, shapefile):
        QMainWindow.__init__(self)
        self.setWindowTitle("Map Viewer")

        canvas = QgsMapCanvas()
        #canvas.useImageToRender(False)
        canvas.setCanvasColor(Qt.white)
        canvas.show()

        layer = QgsVectorLayer(shapefile, "layer1", "ogr")
        if not layer.isValid():
            raise IOError("Invalid shapefile")

        QgsProject.instance().addMapLayer(layer)
        QgsProject.instance().mapLayers()
        # QgsMapLayerRegistry.instance().addMapLayer(layer)
        # canvas.setExtent(layer.extent())
        # canvas.setLayerSet(layer)
        # canvas.setLayerSet([QgsMapCanvasLayer(layer)])

        layout = QVBoxLayout()
        layout.addWidget(canvas)

        contents = QWidget()
        contents.setLayout(layout)
        self.setCentralWidget(contents)



# supply path to qgis install location
QgsApplication.setPrefixPath("/usr/bin/qgis", True)

# create a reference to the QgsApplication, setting the
# second argument to False disables the GUI
qgs = QgsApplication([], True)

# load providers
qgs.initQgis()

# Write your code here to load some layers, use processing
# algorithms, etc.
app = QApplication(sys.argv)

viewer = MapViewer("/home/bot/STATE.shp")
viewer.show()

app.exec_()

# When your script is complete, call exitQgis() to remove the
# provider and layer registries from memory

qgs.exitQgis()

I have tried different shapefiles from different location but all of them raise exception of Invalid shapefile.

My PYTHONPATH:

/usr/share/qgis/python

Output:

Traceback (most recent call last): File "./q_test.py", line 57, in

  viewer = MapViewer("/home/bot/STATE.shp")   File "./q_test.py", line 25, in <strong>init</strong>
  raise IOError("Invalid shapefile") OSError: Invalid shapefile</p>