fix(ambient) actually set the pointer to null when stopping

epicardium/modules/ambient.c

@@ -6,7 +6,7 @@
 #include "gpio.h"
 
 
-#define READ_FREQ 1000
+#define READ_FREQ 100
 
 static uint16_t last_value;
 static TaskHandle_t poll_task;
@@ -67,6 +67,7 @@ void epic_ambient_stop()
         return;
     }
     vTaskDelete(poll_task);
+    poll_task = NULL;
 }
 
 uint16_t epic_ambient_get()