how to remove the error "type error u is null"

DynamicReport.SchemaRelationControl = function (relation) {
 var self =this ;
 var combotable=null;

 var createleftcombo=function (tblname){
    var tables = Ext.create('Ext.data.Store', {
        fields: ['value', 'text'],
        data:[{text:tblname,value:tblname}]
    });
 combotable = Ext.create('Ext.form.ComboBox', {
        fieldLabel: 'lefttable',
        store: tables,
        queryMode: 'local',
        displayField: 'text',
        valueField: 'value',
        renderTo:'fom-container',
    });
    }
     var init=function () {
        createleftcombo(relation.tblname);
    } ();



};
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when i run this code it shows "type error u is null"
please help me