Warning message needed when there is no specific layer instead of python error

In my plugin, when I click a button it will call the findTheCityNames function which is attached below and get features if there is a particular layer. What I want is if there is no specific layer in which it is getting features, it should give a warning message "No Valid layer found" instead python error "List object has no attribute getFeatures".

def findTheCityNames(self):
    self.dlg.Citycombo.clear()
    ct = self.dlg.Statecombo.currentText()
    st = "state='"+ct+"'"
    exp = QgsExpression(st)
    fidAndVal = {}        
    #QMessageBox.information(self.dlg, "info", st) 

    layer = self.iface.activeLayer()

    iterFeat = layer.getFeatures()

    for f in iterFeat:
        val = exp.evaluate(f)
        if bool(val):
            attrs = f.attributes()
            idx = layer.fieldNameIndex('name')
            cityName = f.attributes()[idx]
            fidInt = f.id()
            fidAndVal[fidInt] = cityName

    for x in fidAndVal.keys():
        self.dlg.Citycombo.addItem(fidAndVal[x])