Tkinter RuntimeError: main thread is not in main loop when trying to insert item into listbox from different thread

Question

Whenever I try to insert an item into a tkinter listbox from another thread it gives me this error: RuntimeError: main thread is not in main loop The main parts of my code is down here: Receive Thread:

# receive is running on another thread
def receive(event=None):
    global msg
    global recvinfo
    while True:
        try:
            msg = client_socket.recv(BUFSIZ).decode("utf8")
            decoded = cryptocode.decrypt(msg, PASS)
            if decoded == "User Found":
                recvinfo = 1
            elif decoded == "User Not Found":
                recvinfo = 2
            elif decoded.startswith("gb:"):
                smsg_list.insert(tkinter.END, decoded)
        except OSError:  # Client Side Exited Out
            break

GUI Definition, running in main loop:

def maingui():

    global smsg_list

    receive_thread = Thread(target=receive)
    receive_thread.start()

    smsg_list = tkinter.Listbox(smessages_frame, height="21", width="75",)
    smsg_list.pack(side=tkinter.LEFT, fill=tkinter.BOTH)
    smsg_list.pack()