Tweak for potential null pointer exception seen after play store release

app/src/main/java/nodomain/freeyourgadget/gadgetbridge/devices/banglejs/AppsManagementActivity.java

@@ -143,6 +143,7 @@ public class AppsManagementActivity extends AbstractGBActivity {
         if (webView!=null) webView.post(new Runnable() {
             @Override
             public void run() {
+                if (webView==null) return; // webView may have gone by the time we get called!
                 if (android.os.Build.VERSION.SDK_INT >= android.os.Build.VERSION_CODES.KITKAT) {
                     webView.evaluateJavascript(js, null);
                 } else {