During invasion of Poland Hitler travelled to visit frontline, and as we know, frontline is not particulary safe place. On 5 September Hitler came to met von Kluge near Komierowo, on the same day planes of Polish 42nd Reconnaissance Escadrille dropped bombs 2 kilometers from führer's column. So what if by sheer luck Poles hit the column, killing Hitler as result? When is information of Hitler's death announced to the public? Dead of head of man nr 1 in Third Reich can't be kept secret for long. Invasion of Poland obviously isn't cancelled, but resulting chaos should cause some delays. Still, Poland has no chance to defend itself, all it gets is massive morale boost. Perhaps evacuation of Polish government from Warsaw is cancelled for a while. IMHO despite of Poland's grim military situation once it is confirmed that Hitler is dead that news would be celebrated like declaration of war against Germany by UK and France on 3 September.