B4R Question attachInterrupt

void flow () // Interrupt function
{
   flow_frequency++;
}
void setup()
{
   pinMode(flowsensor, INPUT);
   digitalWrite(flowsensor, HIGH); // Optional Internal Pull-Up
   Serial.begin(9600);
   attachInterrupt(0, flow, RISING); // Setup Interrupt  <-------------- Here I am facing Issue
   sei(); // Enable interrupts   <--------- Here I am facing issue
   currentTime = millis();
   cloopTime = currentTime;
}

#Region Project Attributes
    #AutoFlushLogs: True
    #CheckArrayBounds: True
    #StackBufferSize: 300
#End Region

Sub Process_Globals
    'These global variables will be declared once when the application starts.
    'Public variables can be accessed from all modules.
    Public Serial1 As Serial
    Private pin2 As Pin
    Private timer1 As Timer
    Private counter As UInt
    Dim set= False As Boolean
End Sub

Private Sub AppStart
    Serial1.Initialize(115200)
    Log("AppStart")
    pin2.Initialize(2,pin2.MODE_INPUT_PULLUP)
    pin2.AddListener("pin2_StateChanged")
    timer1.Initialize("timer1_Tick",1000)   
End Sub

Private Sub timer1_Tick()
    Log(counter)
    counter=0
    set= False
End Sub

Private Sub pin2_StateChanged(state As Boolean)
    Select state
        Case True
            counter=counter+1
        Case False
    End Select
End Sub

timer1.enabled=true

   Log(counter/450)

AppStart
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0
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0
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0

Log(counter/450)

#Region Project Attributes
    #AutoFlushLogs: True
    #CheckArrayBounds: True
    #StackBufferSize: 300
#End Region

Sub Process_Globals
    'These global variables will be declared once when the application starts.
    'Public variables can be accessed from all modules.
    Public Serial1 As Serial
    Private pin2 As Pin
    Private timer1 As Timer
    Private counter As UInt
    Private set= False As Boolean
    Private flowrate As Double
    Private totalmililiter=0 As Double
End Sub

Private Sub AppStart
    Serial1.Initialize(115200)
    Log("AppStart")
    pin2.Initialize(2,pin2.MODE_INPUT_PULLUP)
    pin2.AddListener("pin2_StateChanged")
    timer1.Initialize("timer1_Tick",2000)
    timer1.Enabled=True   
End Sub

Private Sub timer1_Tick()
    'Start the math
    'As per the datasheet 450 Pulse per Liter. So we can get 1000/450 or 2.25ML per Pulse.
    flowrate=(counter*2.25)
    flowrate=flowrate*60 'ML(MilliLitres) per minutes
    'Conversion of MilliLitres to Liters
    flowrate=NumberFormat((flowrate/1000),0,2)
    totalmililiter=(totalmililiter+flowrate)
    Log("Flowrate: ",flowrate, " Liter/Minute")
    'Print the cumulative total of litres flowed since starting
    Log("Total Output Liquid Quantity: ", NumberFormat(totalmililiter,0,2))
    'Log("Counter is :",counter)
    counter=0     'Resetting the Counter
    set= False
    
End Sub

Private Sub pin2_StateChanged(state As Boolean)
    Select state
        Case True
            counter=counter+1
        Case False
    End Select
End Sub