Fix exception on generating preview url for installed app screenshot

Some installed apps meant for older server versions might unexpectedly
offer up screenshot values in a non-string format (e.g. health). Avoid
an exception by checking first if the first app screenshot is indeed a
string and only then trying to create the $proxyPreviewUrl

Signed-off-by: Claudio Cambra <developer@claudiocambra.com>

Author: claucambra

apps/settings/lib/Controller/AppSettingsController.php

@@ -245,9 +245,11 @@ private function fetchApps() {
 		$apps = $appClass->listAllApps();
 		foreach ($apps as $app) {
 			$app['installed'] = true;
+			$appScreenshot = $app['screenshot'][0];
 			// locally installed apps have a flatted screenshot property
-			if (isset($app['screenshot'][0])) {
-				$app['screenshot'] = $this->createProxyPreviewUrl($app['screenshot'][0]);
+			if (is_string($appScreenshot)) {
+				$proxyPreviewUrl = $this->createProxyPreviewUrl($appScreenshot);
+				$app['screenshot'] = $proxyPreviewUrl;
 			}
 			$this->allApps[$app['id']] = $app;
 		}