allow play to be pressed if stopped but playlist not empty

src/playbin.vala

@@ -237,7 +237,12 @@ public class Playbin : GLib.Object {
     }
 
     public void play () {
-        assert (this.state != PlaybinState.STOPPED);
+        if (this.state == PlaybinState.STOPPED) {
+            // allow only when playlist is not empty
+            // and start from the top
+            assert (this._play_queue.get_n_items () > 0);
+            this.select_track (0);
+        } else {
             this.state = PlaybinState.PLAYING;
             debug ("setting state to playing");
             var ret = this.mpv.set_property_flag("pause", false);
@@ -245,6 +250,7 @@ public class Playbin : GLib.Object {
                 debug (@"failed to set state to playing ($(ret)): $(ret.to_string())");
             }
         }
+    }
 
     public void go_to_next_track ()
             requires (this.state != PlaybinState.STOPPED)

src/ui/playbar.blp

@@ -119,7 +119,7 @@ template $UiPlaybar: Box {
         Button {
           icon-name: bind $play_pause_icon_name (template.playbin as <$Playbin>.state as <$PlaybinState>) as <string>;
           valign: center;
-          sensitive: bind $playbin_active (template.playbin as <$Playbin>.state as <$PlaybinState>) as <bool>;
+          sensitive: bind $can_press_play (template.playbin as <$Playbin>.state as <$PlaybinState>, template.playbin as <$Playbin>.play_queue_position) as <bool>;
 
           clicked => $on_play_pause_clicked ();
         }

src/ui/playbar.vala

@@ -41,6 +41,10 @@ class Ui.Playbar : Gtk.Box {
         return state != PlaybinState.STOPPED;
     }
 
+    [GtkCallback] private bool can_press_play (PlaybinState state, uint n_items) {
+        return state != PlaybinState.STOPPED || n_items > 0;
+    }
+
     [GtkCallback] private string mute_button_icon_name (bool mute) {
         return mute ? "audio-volume-muted" : "audio-volume-high";
     }