Fix runtime warnings

[ryonakano]

Fixes the following warnings:

• On pressing "Add Connection" button in the Hosts welcome screen:

(com.github.muriloventuroso.easyssh:2): Gtk-CRITICAL **: 09:01:13.067: gtk_grid_attach: assertion '_gtk_widget_get_parent (child) == NULL' failed

• On pressing "Add Accounts" button in the Accounts welcome screen:

(com.github.muriloventuroso.easyssh:2): Gtk-CRITICAL **: 09:02:20.747: gtk_widget_get_style_context: assertion 'GTK_IS_WIDGET (widget)' failed

(com.github.muriloventuroso.easyssh:2): Gtk-CRITICAL **: 09:02:20.747: gtk_style_context_add_class: assertion 'GTK_IS_STYLE_CONTEXT (context)' failed

(com.github.muriloventuroso.easyssh:2): Gtk-CRITICAL **: 09:02:20.747: gtk_container_add: assertion 'GTK_IS_WIDGET (widget)' failed

• On pressing the search icon in the headerbar when you have no terminal open (e.g. in the welcome screen):

(com.github.muriloventuroso.easyssh:2): Gtk-CRITICAL **: 09:03:34.260: gtk_widget_grab_focus: assertion 'GTK_IS_WIDGET (widget)' failed

• On removing the last remaining localhost tab:

** (com.github.muriloventuroso.easyssh:2): CRITICAL **: 09:06:16.337: granite_widgets_tab_set_icon: assertion 'self != NULL' failed