Error: Invalid Data. 
Review all error messages below to correct your data.
Apex trigger ProjectDuplicateCheck caused an unexpected exception, contact your administrator: ProjectDuplicateCheck: execution of BeforeUpdate caused by: System.QueryException: Non-selective query against large object type (more than 200000 rows). Consider an indexed filter or contact salesforce.com about custom indexing. Even if a field is indexed a filter might still not be selective when: 1. The filter value includes null (for instance binding with a list that contains null) 2. Data skew exists whereby the number of matching rows is very large (for instance, filtering for a particular foreign key value that occurs many times): Trigger.ProjectDuplicateCheck: line 11, column 1

Trigger:-
trigger ProjectDuplicateCheck on MC2__Project__c (before insert,before update) {
   
    Set<String> setProjId = new Set<String>();
    For(MC2__Project__c mp : trigger.new)
    {
        setProjId.add(mp.MC2__MC_Project_ID__c);
    }
    
    if(setProjId.size() > 0 )
    { 
        List<MC2__Project__c> lstproject = [select MC2__MC_Project_ID__c,id from MC2__Project__c where MC2__MC_Project_ID__c!=null AND MC2__MC_Project_ID__c in:setProjId];
        
        
        Map<String,MC2__Project__c> Projmap = new Map<String,MC2__Project__c>();
        For(MC2__Project__c mhp: lstproject)
        {
            Projmap.put(mhp.MC2__MC_Project_ID__c,mhp);
        }
        
        For(MC2__Project__c mhp : trigger.new)
        {
            if(Projmap.containsKey(mhp.MC2__MC_Project_ID__c))
            {
                mhp.MC2__MC_Project_ID__c.addError('Project Id already exists, Please create project with new Project Id');
            }
        } 
      }
}