MainWindow: Fix assertion message about choose_project_button

[ryonakano]

Fixes the following messages on launch:

user@veos7d:~/code/builddir$ io.elementary.code 

** (io.elementary.code:10279): CRITICAL **: 11:55:50.768: code_sidebar_get_choose_project_button: assertion 'self != NULL' failed

(io.elementary.code:10279): GLib-GObject-WARNING **: 11:55:50.768: invalid (NULL) pointer instance

(io.elementary.code:10279): GLib-GObject-CRITICAL **: 11:55:50.768: g_signal_connect_object: assertion 'G_TYPE_CHECK_INSTANCE (instance)' failed

[ryonakano]

Closing in favor of #1250