Changes NULL to nullptr

Changed an instance of NULL to nullptr in order to compile with PlatformIO.

src/IBusBM.h

@@ -90,7 +90,7 @@ class IBusBM {
   } sensorinfo;
   sensorinfo sensors[SENSORMAX];
   uint8_t NumberSensors = 0;        // number of sensors
-  IBusBM* IBusBMnext = NULL;        // pointer to the next class instance to be used to call the loop() method from timer interrupt
+  IBusBM* IBusBMnext = nullptr;        // pointer to the next class instance to be used to call the loop() method from timer interrupt
 };
 
 #endif