openLoaded method NPE check

TASMAN-webapp/src/main/java/it/inaf/ia2/tsm/webapp/TapSchemaLoader.java

@@ -184,6 +184,12 @@ public class TapSchemaLoader implements Serializable {
     }
 
     public String openLoaded() {
+        
+        if (loadedTapSchema == null) {
+            throw new IllegalStateException("Attempted to access a TAP_SCHEMA not loaded yet. "
+                    + "This may be caused by an error on frontend or a browser cache issue.");
+        }
+        
         ConsistencyChecks checks = loadedTapSchema.getConsistencyChecks();
         if (checks != null && (checks.isInconsistent() || checks.isHasWarnings())) {
             consistencyChecksBean.setTapSchema(loadedTapSchema);