I have a web application I have been working with and creating automations. I have hit a block on on a specific step where I click a button, which then opens a popup. On this popup there is only a dropdown and a button to submit. I am unable to manipulate this button and dropdown

I think this might be a modal container. I have attached a copy of the code for the page as it was too much to just paste in here.
I have looked into tutorials for modals and tried a number of things, but so far, I am still having some challenges here. I pasted the code below and also attached a file for easier reading
I'm a little stumped as to how to proceed. Does anyone have any thoughts?
//NEW PAGE - FINISH RAW MATERIAL SETUP
//Raw Material Mill Selection
//WebElement rMatMill=driver.findElement(By.id("bom-common-rawmaterialscreencreate-raw-material-mill-button"));
WebElement rMatMill=driver.findElement(By.xpath("/html//button[@id='bom-common-rawmaterialscreencreate-raw-material-mill-button']"));
Thread.sleep(1000);
rMatMill.click();
//(By.xpath("/html//select[contains(@id,'bom-common-rawmaterialscreen-calculationtype')]"));
WebElement modalContainer = driver.findElement(By.className("modalBlockout"));
//WebElement modalContentBody = driver.findElement(By.xpath("/html//div[@class='modalHost ui-front']/div"));
WebElement modalContentBody = driver.findElement(By.className("ui-dialog ui-widget ui-widget-content ui-corner-all ui-dialog-buttons tixui5-modal-dialog ui-draggable ui-resizable"));
WebElement modalRawMatMill = modalContentBody.findElement(By.xpath("/html//select[contains(@id,'bom-common-dialogs-copyrawmaterialmilldialogmillcode-combo')]"));
Select modalRawMatMillDD = new Select(modalRawMatMill);
modalRawMatMillDD.selectByIndex(0);
//modalRawMatMillDD.click();
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