Well you can optimize your code a bit. Have a checking for parentId or using try catch

public class VFFileUpload {
    public Id recId {
        get;
        set;
    }

    public VFFileUpload(ApexPages.StandardController ctlr) {
        recId = ctlr.Id();
      	myAttachment = new Attachment();
    }

    public Attachment myAttachment {
        get;
        set;
    }
	

    public PageReference UploadFile() {
        PageReference pr;
        if (fileBody != null && fileName != null) {
          try{
            myAttachment.ParentId = recId;
            insert myAttachment;           
            pr = new PageReference('/' + myAttachment.Id);
            pr.setRedirect(true);
            return pr;
          }
          catch(Exception ex){
          	Apexpages.addmessages(ex);
          }
        }
		myAttachment = new Attachment(); //incase of error reset the variable to avoid view state errors
        return pr;
    }
}

<apex:page standardController="MyCustomObject__c" extensions="VFFileUpload">
    <apex:form>
        <apex:pageBlock title="Upload Attachment">
            <apex:inputFile style="width:100%" id="fileToUpload" value="{!myAttachment.Body}" filename="{!myAttachment.Name}" />
            <apex:commandButton value="Upload Attachment" action="{!UploadFile}" />
        </apex:pageBlock>
    </apex:form>
</apex:page>

 Main cause : Probably you are not passing the record Id to the page. Page URL Should be /apex/VFFileUploadPage?id=<MyCustomObject__c.Id>

Replace <MyCustomObject__c.Id> with a record ID