Fix defaultPlugins logic to parse the incoming value

src/install.js

@@ -478,17 +478,24 @@ function enableDefaultPlugins(next) {
 	process.stdout.write('Enabling default plugins\n');
 
 	var defaultEnabled = [
-		'nodebb-plugin-composer-default',
-		'nodebb-plugin-markdown',
-		'nodebb-plugin-mentions',
-		'nodebb-widget-essentials',
-		'nodebb-rewards-essentials',
-		'nodebb-plugin-soundpack-default',
-		'nodebb-plugin-emoji-extended'
-	];
-
-	if (Array.isArray(nconf.get('defaultPlugins'))) {
-		defaultEnabled = defaultEnabled.concat(nconf.get('defaultPlugins'));
+			'nodebb-plugin-composer-default',
+			'nodebb-plugin-markdown',
+			'nodebb-plugin-mentions',
+			'nodebb-widget-essentials',
+			'nodebb-rewards-essentials',
+			'nodebb-plugin-soundpack-default',
+			'nodebb-plugin-emoji-extended'
+		],
+		customDefaults = nconf.get('defaultPlugins');
+
+	if (customDefaults && customDefaults.length) {
+		try {
+			customDefaults = JSON.parse(customDefaults);
+			defaultEnabled = defaultEnabled.concat(customDefaults);
+		} catch (e) {
+			// Invalid value received
+			winston.warn('[install/enableDefaultPlugins] Invalid defaultPlugins value received. Ignoring.');
+		}
 	}
 
 	defaultEnabled = defaultEnabled.filter(function(plugin, index, array) {