Prevent passing null to htmlspecialchars

application/views/search/main.php

@@ -27,7 +27,7 @@
 		  <div class="form-group row">
 		    <label for="callsign" class="col-sm-2 col-form-label">Callsign / Gridsquare</label>
 		    <div class="col-sm-8">
-		      <input type="text" class="form-control" id="callsign" value="<?php echo htmlspecialchars($this->input->post('callsign')); ?>">
+		      <input type="text" class="form-control" id="callsign" value="<?php if ($this->input->post('callsign') !== null) { echo htmlspecialchars($this->input->post('callsign')); }; ?>">
 		    </div>
 		    <div class="col-sm-2">
 		    	<button onclick="searchButtonPress()" class="btn btn-outline-success my-2 my-sm-0" type="submit"><i class="fas fa-search"></i> Search</button>