Help with python 'not subscriptable' error

[Extra]

Hello,

I have this chunk of code from my PyQt GUI that gets a result from a SQLite table and uses that result to do a math calculation.

Snippet:

else: 
                        #Connect to the inventory database (inventory.db)
                        connection = sqlite3.connect(InventoryDatabase)
                        cursor = connection.cursor()
                        cursor.execute("SELECT Sell_Price_$ FROM items WHERE Name = ?",(SelectedItemName,))
                        connection.commit()
                        SellPrice = cursor.fetchone()
                        
                        # #Update the quantity
                        # cursor.execute("UPDATE items SET Quantity = Quantity - ? WHERE Name = ?",(SelectedItemQuantity, SelectedItemName,))
                        # connection.commit()
                        
                        #Close the connection
                        connection.close()

                        print('Selcted Item Quantity: ', SelectedItemQuantity)

                        x = float(SelectedItemQuantity)
                        print('Float Value = ', x)
                        y = SellPrice[0]
                        print("y = ",y)
                        z = float(y)
                        print('x*z= ', x*z)
                        MaterialUsedPrice2 = x*z

                        #Calculate the Price of Material Used
                        #MaterialUsedPrice = SelectedItemQuantity * SellPrice
                        print('The Total Price for the', '(',x,')', SelectedItemName, ' Used is: $', MaterialUsedPrice2)

Output:

Output:
2 , 1/2" PVC Coupling
('PVC',)
Selcted Item Quantity:  2
Float Value =  2.0
y =  1.5
x*z=  3.0
The Total Price for the ( 2.0 ) 1/2" PVC Coupling  Used is: $ 3.0

#I don't know why this gets printed (again...Since it's already been printed above (Selcted Item Quantity:  2
Float Value =  2.0) and I don't know why the values are 0)
0 , -
None
Selcted Item Quantity:  0
Float Value =  0.0

y = SellPrice[0]
TypeError: 'NoneType' object is not subscriptable

I get this error and I don't know why (When I do print("y = ",y) I get 1.5, which is the correct value, so why am I getting this error?)

Error:
y = SellPrice[0]
TypeError: 'NoneType' object is not subscriptable

Why do I get this error and how do I fix it?

Thanks in advance.

Full function (If it helps):

    def SubmitClicked(self):
        #Get User Inputted Quantity & Material
        #Probably going to have to do it the long way: Do all 22 comboBoxes individually. 
        for widget in self.MaterialUsedFrame.children():        
            if isinstance(widget, QSpinBox):
                    SelectedItemQuantity = widget.text()
            if isinstance(widget,QComboBox):
                    SelectedItemName = widget.currentText()
                    print(SelectedItemQuantity, ',' ,SelectedItemName)
        
                    #Connect to the inventory database (inventory.db)
                    connection = sqlite3.connect(InventoryDatabase)
                    cursor = connection.cursor()
                    cursor.execute("SELECT Main_Category FROM items WHERE Name = ?",(SelectedItemName,))
                    connection.commit()
                    Result = cursor.fetchone()
                    #Close the connection
                    connection.close()
                    print(Result)

                    if Result == ('Wire',):
                        #Connect to the inventory database (inventory.db)
                        connection = sqlite3.connect(InventoryDatabase)
                        cursor = connection.cursor()
                        cursor.execute("SELECT Price_Per_Ft FROM items WHERE Name = ?",(SelectedItemName,))
                        connection.commit()
                        PricePerFt = cursor.fetchone()
                        #Close the connection
                        connection.close()
                        print(PricePerFt)
                        
                        #Might be better off making all those SpinBoxes Labels
                        #& Just setting the text to the price
                        for widget in self.MaterialPriceFrame.children():        
                            if isinstance(widget, QDoubleSpinBox):
                                widget.setValue(80.80)

                    else: 
                        #Connect to the inventory database (inventory.db)
                        connection = sqlite3.connect(InventoryDatabase)
                        cursor = connection.cursor()
                        cursor.execute("SELECT Sell_Price_$ FROM items WHERE Name = ?",(SelectedItemName,))
                        connection.commit()
                        SellPrice = cursor.fetchone()
                        
                        # #Update the quantity
                        # cursor.execute("UPDATE items SET Quantity = Quantity - ? WHERE Name = ?",(SelectedItemQuantity, SelectedItemName,))
                        # connection.commit()
                        
                        #Close the connection
                        connection.close()

                        print('Selcted Item Quantity: ', SelectedItemQuantity)

                        x = float(SelectedItemQuantity)
                        print('Float Value = ', x)
                        y = SellPrice[0]
                        print("y = ",y)
                        z = float(y)
                        print('x*z= ', x*z)
                        MaterialUsedPrice2 = x*z

                        #Calculate the Price of Material Used
                        #MaterialUsedPrice = SelectedItemQuantity * SellPrice
                        print('The Total Price for the', '(',x,')', SelectedItemName, ' Used is: $', MaterialUsedPrice2)
#----------------------------------------------------------------------------------------------------