How to stretch tabcontrol to the bottom.

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C#

XAML

WPF

Hello I'm making a program that has a toolbar and underneath it a tab control.In my designer it shows like this:
Image[^]
Exactly like i want. (the button is just there for testing).
But when I launch the application the tab controls shrink.
Image[^]

How can I fix this?

EDIT:
sorry i forgot the code

<pre><Page x:Class="InvoiceX.pages.InvoiceEditor"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:local="clr-namespace:InvoiceX.pages"
      mc:Ignorable="d" 
      d:DesignHeight="450" d:DesignWidth="800"
      Title="InvoiceEditor">

    <Grid VerticalAlignment="Stretch">
        <Grid.RowDefinitions>
            <RowDefinition Height="5*" />
            <RowDefinition Height="85*" />
        </Grid.RowDefinitions>
        
        <ToolBar ToolBarTray.IsLocked="true" Grid.Row="0">
                <Menu>
                    <MenuItem Header="File">
                        <Button Command="New" Content="New" />
                        <Button Command="Open" Content="Open" />
                        <Button Command="Save" Content="Save" />
                    </MenuItem>
                </Menu>
         </ToolBar>

        <TabControl x:Name="InvoiceNavigation" Grid.Row="1" >
            <TabItem Header="Customer">
                <Grid Background="#FFE5E5E5">
                    <Button Command="Save" Content="Save"/>
                </Grid>

            </TabItem>
            <TabItem Header="Items">
                <Grid Background="#FFE5E5E5"/>
            </TabItem>
            <TabItem Header="Invoice">
                <Grid Background="#FFE5E5E5">
                </Grid>
            </TabItem>
        </TabControl>

    </Grid>
    
</Page>

What I have tried:

Searching the internet but I did not find anything that could help me in this situation.