"Load Binary" does nothing

**Describe the bug**

After path to a binary typed-in, and Load Binary button pressed, nothing is happening.

**To Reproduce**

1. In terms of terminal commands:

```
λ cd /tmp
λ cat test3.c
#include

int main() {
puts("Hey!");
}
λ gcc test3.c -o a -g3 -O0
λ gdbgui
Opening gdbgui with default browser at http://127.0.0.1:5000
View gdbgui dashboard at http://127.0.0.1:5000/dashboard
exit gdbgui by pressing CTRL+C
The client is using an unsupported version of the Socket.IO or Engine.IO protocols (further occurrences of this error will be logged with level INFO)
(2021-03-01 10:01:57,948) The client is using an unsupported version of the Socket.IO or Engine.IO protocols (further occurrences of this error will be logged with level INFO)
```

2. Now, in the browser window that `gdbgui` has opened, type `/tmp/a` path to binary, and press Load Binary button.

**Expected behavior**

Source code appears in widget that currently says `no source code or assembly to display`

**Actual behavior**

Nothing is happening at all.

**Screenshots**


**Please complete the following information:**
* OS: Archlinux
* gdbgui version (`gdbgui -v`): 0.14.0.2
* gdb version (`gdb -v`): 10.1
* browser: `chromium` and `qutebrowser`

**Additional context**

Idk if that's relevant, but upon starting up `gdbserver` complains *(as can be seen in the steps-to-reproduce)*

```
(2021-03-01 10:01:57,948) The client is using an unsupported version of the Socket.IO or Engine.IO protocols (further occurrences of this error will be logged with level INFO)
```

I suppose there's a PR for that: https://github.com/cs01/gdbgui/pull/375