fix crashes on pattern change when number of track does ot match (fix #…

…15)

ui/ui.go

@@ -91,6 +91,8 @@ func (m mainModel) Init() tea.Cmd {
 }
 
 func (m mainModel) Update(msg tea.Msg) (tea.Model, tea.Cmd) {
+	m.resetPatternState()
+
 	switch msg := msg.(type) {
 
 	case tea.WindowSizeMsg:
@@ -359,6 +361,16 @@ func (m mainModel) View() string {
 	)
 }
 
+// resetPattern ensures that we reset active track and step state
+// after a pattern chain if needed
+func (m *mainModel) resetPatternState() {
+	if m.activeTrack >= len(m.seq.Tracks()) || m.activeStep >= len(m.seq.Tracks()[m.activeTrack].Steps()) {
+		m.activeTrack = 0
+		m.activeTrackPage = 0
+		m.activeStep = 0
+	}
+}
+
 func (m *mainModel) getActiveTrack() sequencer.Track {
 	return m.seq.Tracks()[m.activeTrack]
 }