serial: improve nil check

mainunit.lfm

@@ -22,7 +22,7 @@ object MainForm: TMainForm
     Height = 580
     Top = 10
     Width = 940
-    ActivePage = TabSheet2
+    ActivePage = SerialTabSheet
     Anchors = [akTop, akLeft, akRight, akBottom]
     BorderSpacing.Left = 10
     BorderSpacing.Top = 10
@@ -34,7 +34,7 @@ object MainForm: TMainForm
     Font.Pitch = fpVariable
     Font.Quality = fqDraft
     ParentFont = False
-    TabIndex = 1
+    TabIndex = 0
     TabOrder = 0
     object SerialTabSheet: TTabSheet
       Caption = '串口'

mainunit.pas

@@ -5,8 +5,8 @@ unit mainunit;
 interface
 
 uses
-    Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls, ExtCtrls,
-    ComCtrls, Spin, MaskEdit, SpinEx, Synaser, SerialManager, Types,
+    Classes, SysUtils, Forms, Controls, Graphics, Dialogs, ExtCtrls,
+    ComCtrls, MaskEdit, SpinEx, Synaser, SerialManager,
     SerialPortFrameUnit, TCPFrameUnit;
 
 type

serialmanager.pas

@@ -69,21 +69,26 @@ end;
 
 procedure SerialThread.Close;
 begin
-    MserialPort.Purge();
-    MserialPort.CloseSocket();
-    Terminate();
-    FreeAndNil(MserialPort);
-    ProcessOnCloseEvent();
+    if MserialPort <> nil then
+    begin
+        MserialPort.Purge();
+        MserialPort.CloseSocket();
+        Terminate();
+        FreeAndNil(MserialPort);
+        ProcessOnCloseEvent();
+    end;
 end;
 
 procedure SerialThread.SendString(Data: ansistring);
 begin
-    MserialPort.SendString(Data);
+    if (MserialPort <> nil) and isConnected() then
+        MserialPort.SendString(Data);
 end;
 
 function SerialThread.SendBuffer(buffer: pointer; length: integer): integer;
 begin
-    Result := MserialPort.SendBuffer(buffer, length);
+    if (MserialPort <> nil) and isConnected() then
+        Result := MserialPort.SendBuffer(buffer, length);
 end;
 
 procedure SerialThread.ConfigureSerial(baud, bits: integer; parity: char;