Fix missing sidebar.php

sidebar.php is required by WordPress core since version 3.0.0

sidebar.php

@@ -0,0 +1,17 @@
+<?php
+/**
+ * The sidebar containing the main widget area.
+ *
+ * @package understrap
+ */
+
+if ( ! is_active_sidebar( 'sidebar-1' ) ) {
+	return;
+}
+?>
+
+<div class="col-md-4 widget-area" id="secondary" role="complementary">
+
+	<?php dynamic_sidebar( 'sidebar-1' ); ?>
+
+</div><!-- #secondary -->