Make checkAtmosphereSupport() non-static (#12131)

Fixes: https://github.com/vaadin/multiplatform-runtime/issues/77

server/src/main/java/com/vaadin/server/VaadinService.java

     private Iterable<DependencyFilter> dependencyFilters;
     private ConnectorIdGenerator connectorIdGenerator;
 
-    private boolean atmosphereAvailable = checkAtmosphereSupport();
+    private Boolean atmosphereAvailable = null;
 
     /**
      * Keeps track of whether a warning about missing push support has already
      *         is not available.
      */
     public boolean ensurePushAvailable() {
-        if (atmosphereAvailable) {
+        if (isAtmosphereAvailable()) {
             return true;
         } else {
             if (!pushWarningEmitted) {
         }
     }
 
-    private static boolean checkAtmosphereSupport() {
+    private boolean checkAtmosphereSupport() {
         String rawVersion = AtmospherePushConnection.getAtmosphereVersion();
         if (rawVersion == null) {
             return false;
      * @return true if Atmosphere is available, false otherwise
      */
     protected boolean isAtmosphereAvailable() {
+        if (atmosphereAvailable == null) {
+            atmosphereAvailable = checkAtmosphereSupport();
+        }
         return atmosphereAvailable;
     }