Activity doesn't accept signals

Question

I work with following workflow i get following exception:

org.activiti.engine.ActivitiException: this activity doesn't accept signals

where did i mistake in the following workflow.

Actually in my workflow task goes to one group,this group is completed the task with in time it goes to next group,if not mail goes to manager of group whose not completed task with in the time.

<definitions xmlns="http://www.omg.org/spec/BPMN/20100524/MODEL" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:activiti="http://activiti.org/bpmn" xmlns:bpmndi="http://www.omg.org/spec/BPMN/20100524/DI" xmlns:omgdc="http://www.omg.org/spec/DD/20100524/DC" xmlns:omgdi="http://www.omg.org/spec/DD/20100524/DI" typeLanguage="http://www.w3.org/2001/XMLSchema" expressionLanguage="http://www.w3.org/1999/XPath" targetNamespace="http://www.activiti.org/test">
<process id="mailFlow" name="My process" isExecutable="true">
    <startEvent id="startevent1" name="Start"></startEvent>
    <scriptTask id="scripttask1" name="Script Task" scriptFormat="groovy" activiti:autoStoreVariables="true">
        <script>out:println "This Category Name:"+incidentcategory;</script>
    </scriptTask>
    <sequenceFlow id="flow1" sourceRef="startevent1" targetRef="scripttask1">
        <conditionExpression xsi:type="tFormalExpression"><![CDATA[${incidentcategory=='BAT'}]]></conditionExpression>
    </sequenceFlow>
    <userTask id="usertask1" name="User Task A" activiti:candidateGroups="Group A"></userTask>
    <sequenceFlow id="flow2" sourceRef="scripttask1" targetRef="usertask1"></sequenceFlow>

    <boundaryEvent id="boundarytimer1" name="Timer" attachedToRef="usertask1" cancelActivity="true">
        <timerEventDefinition>
            <timeDuration>PT40S</timeDuration>
        </timerEventDefinition>
    </boundaryEvent>

    <serviceTask id="mailtask1" name="Mail Task A" activiti:type="mail">
        <extensionElements>
            <activiti:field name="to">
                <activiti:string>xxxx@gmail.com</activiti:string>
            </activiti:field>
            <activiti:field name="html">
                <activiti:string>&lt;html&gt;
                &lt;body&gt;
                Hello ,&lt;br/&gt;&lt;br/&gt;
                This from Mail Task A&lt;br/&gt;&lt;br/&gt;
                Your problem was solved.&lt;br/&gt;&lt;br/&gt;
                Kind regards,&lt;br/&gt;
                Airtel ltd.
                &lt;/body&gt;
                &lt;/html&gt;</activiti:string>
            </activiti:field>
            <activiti:field name="subject">
                <activiti:string>Task Pending from Group A</activiti:string>
            </activiti:field>
        </extensionElements>
    </serviceTask>

    <sequenceFlow id="flow3" sourceRef="usertask1" targetRef="mailtask1"></sequenceFlow>
    <userTask id="usertask2" name="User Task B" activiti:candidateGroups="Group B"></userTask>

    <boundaryEvent id="boundarytimer2" name="Timer" attachedToRef="usertask2" cancelActivity="true">
        <timerEventDefinition>
            <timeDuration>PT20S</timeDuration>
        </timerEventDefinition>
    </boundaryEvent>

    <serviceTask id="mailtask2" name="Mail Task B" activiti:type="mail">
        <extensionElements>
            <activiti:field name="to">
                <activiti:string>xxxx@gmail.com</activiti:string>
            </activiti:field>
            <activiti:field name="html">
                <activiti:string>&lt;html&gt;
                &lt;body&gt;
                Hello ,&lt;br/&gt;&lt;br/&gt;
                This from Mail Task B&lt;br/&gt;&lt;br/&gt;
                Your problem was solved.&lt;br/&gt;&lt;br/&gt;
                Kind regards,&lt;br/&gt;
                Airtel ltd.
                &lt;/body&gt;
                &lt;/html&gt;</activiti:string>
            </activiti:field>
            <activiti:field name="subject">
                <activiti:string>Task Pending from Group B</activiti:string>
            </activiti:field>
        </extensionElements>
    </serviceTask>

    <sequenceFlow id="flow5" sourceRef="boundarytimer2" targetRef="mailtask2"></sequenceFlow>
    <userTask id="usertask3" name="User Task C" activiti:candidateGroups="Group C"></userTask>

    <serviceTask id="mailtask3" name="Mail Task C" activiti:type="mail">
        <extensionElements>
            <activiti:field name="to">
                <activiti:string>xxxx@gmail.com</activiti:string>
            </activiti:field>
            <activiti:field name="html">
                <activiti:string>&lt;html&gt;
                &lt;body&gt;
                Hello ,&lt;br/&gt;&lt;br/&gt;
                This from Mail Task C&lt;br/&gt;&lt;br/&gt;
                Your problem was solved.&lt;br/&gt;&lt;br/&gt;
                Kind regards,&lt;br/&gt;
                Airtel ltd.
                &lt;/body&gt;
                &lt;/html&gt;</activiti:string>
            </activiti:field>
            <activiti:field name="subject">
                <activiti:string>Task Completed from Group C</activiti:string>
            </activiti:field>
        </extensionElements>
    </serviceTask>

    <sequenceFlow id="flow7" sourceRef="usertask3" targetRef="mailtask3"></sequenceFlow>
    <endEvent id="endevent1" name="End"></endEvent>
    <sequenceFlow id="flow10" sourceRef="mailtask3" targetRef="endevent1"></sequenceFlow>
    <sequenceFlow id="flow11" sourceRef="usertask1" targetRef="usertask2"></sequenceFlow>
    <sequenceFlow id="flow12" sourceRef="usertask2" targetRef="usertask3"></sequenceFlow>
</process>

Answer 1

Sorry guys in my workflow i made a mistake at first user task A timer boundary event.Actually i supposed to give from user task A time boundary event ===> mail Task A,but i made a mistake i assigned from user task A ===> mail Task A while dragging sequence.Now my problem solved. But there is any situations org.activiti.engine.ActivitiException: this activity doesn't accept signals this exception occurs could you please tell me.