Fix defaultPlugins logic to parse the incoming value

src/install.js

@@ -485,10 +485,17 @@ function enableDefaultPlugins(next) {
 			'nodebb-rewards-essentials',
 			'nodebb-plugin-soundpack-default',
 			'nodebb-plugin-emoji-extended'
-	];
+		],
+		customDefaults = nconf.get('defaultPlugins');
 
-	if (Array.isArray(nconf.get('defaultPlugins'))) {
+	if (customDefaults && customDefaults.length) {
-		defaultEnabled = defaultEnabled.concat(nconf.get('defaultPlugins'));
+		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) {