Check RTSP stream selection control is valid

Add a check to make sure the RTSP Stream control is valid before adding the 'onchange' event to avoid breaking the other controls

scripts/spectrogram.php

@@ -481,6 +481,7 @@ h1 {
 
 <script>
 var rtsp_stream_select = document.getElementById("rtsp_stream_select");
+if (typeof (rtsp_stream_select) !== 'undefined' && rtsp_stream_select !== null) {
     //When the dropdown selection is changed set the new value is settings, then restart the livestream service so it broadcasts newly selected RTSP stream
     rtsp_stream_select.onchange = function () {
         if (this.value !== 'undefined') {
@@ -524,6 +525,7 @@ rtsp_stream_select.onchange = function() {
             }
         }
     }
+}
 
 var slider = document.getElementById("gain_input");
 var output = document.getElementById("gain_value");