Fix NPE in FluxWindowTimeout when window is null

Add null check in WindowTimeoutWithBackpressureSubscriber.onNext()
to handle hot publishers that emit before downstream request().

Elements are dropped gracefully using Operators.onNextDropped()
when window hasn't been created yet, maintaining backpressure semantics.

Signed-off-by: AHWSWTYCL <2190844544@qq.com>

Author: albert21wang

reactor-core/src/main/java/reactor/core/publisher/FluxWindowTimeout.java

@@ -184,6 +184,11 @@ public void onNext(T t) {
 				}
 
 				final InnerWindow<T> window = this.window;
+				if (window == null) {
+					Operators.onDiscard(t, this.actual.currentContext());
+					return;
+				}
+
 				if (window.sendNext(t)) {
 					return;
 				}